Marla J. Koberstein

Master‘s thesis

Expansion of the brown shrimp Crangon crangon L. onto juvenile plaice Pleuronectes platessa L. nursery habitat in the Westfjords of Iceland

Marla J. Koberstein

Advisor: Jόnas Páll Jόnasson

University of Akureyri Faculty of Business and Science University Centre of the Westfjords Master of Resource Management: Coastal and Marine Management Ísafjörður, February 2013

Supervisory Committee

Advisor: Name, title

Reader: Name, title

Program Director: Dagný Arnarsdóttir, MSc.

Marla Koberstein Expansion of the brown shrimp Crangon crangon L. onto juvenile plaice Pleuronectes platessa L. nursery habitat in the Westfjords of Iceland 45 ECTS thesis submitted in partial fulfillment of a Master of Resource Management degree in Coastal and Marine Management at the University Centre of the Westfjords, Suðurgata 12, 400 Ísafjörður, Iceland Degree accredited by the University of Akureyri, Faculty of Business and Science, Borgir, 600 Akureyri, Iceland

Printing: Háskólaprent, Reykjavik, February 2013

Declaration

I hereby confirm that I am the sole author of this thesis and it is a product of my own academic research.

______Student‘s name

Abstract

Sandy-bottom coastal ecosystems provide integral nursery habitat for juvenile fishes, and threats to these regions compromise populations at this critical life stage. The threat of aquatic invasive species in particular can be difficult to detect, and climate change may facilitate the spread and establishment of new species. In 2003, the European brown shrimp Crangon crangon L. was discovered off the southwest coast of Iceland. This species is a concern for Iceland due to the combination of its dominance in coastal communities and level of predation on juvenile flatfish, namely plaice Pleuronectes platessa L., observed in its native range. The purpose of this study was to 1) determine the expansion of the brown shrimp in the Westfjords and map its relative density and 2) determine the role the Westfjords plays in providing essential nursery habitat to juvenile Icelandic plaice, a commercial fishery in Iceland that has experienced a decline in landings in recent years. Research goals were achieved by collecting juvenile plaice and brown shrimp using a beam trawl at eleven sampling stations between July 24 and July 31, 2012. Results indicate a northward expansion of the brown shrimp as far as Bolungarvík. The highest density (26.67 ind. per 100m2), was found at Brjánslækur. Multiple generations of the brown shrimp were also observed, indicating reproducing populations. High densities (greater than 200 ind. per 100m2) of 0-group plaice were also found at three sites. These two findings offer a preliminary assessment of the potential risk the brown shrimp may pose to 0-group plaice, and establish the need for continued monitoring of the brown shrimp at these stations. Further, this study has identified essential juvenile plaice habitat in the Westfjords for the success of the Icelandic plaice fishery.

Útdráttur

Sandfjörur gegna mikilvægu hlutverki sem uppeldisslóðir fyrir ungviði fiska. Öll röskun á slíkum svæðum er líkleg til að skaða stofnana á viðkvæmum lífsstigum. Erfitt er að greina áhrif af ágengum tegundum í sjó en hlýnandi loftslag getur auðveldað flutning og viðgang nýrra tegunda. Árið 2003 varð fyrst vart við sandrækju Crangon crangon L.við Íslandsstrendur. Rétt er að gefa henni gaum því í upprunalegum heimkynnum er sandrækjan einkennistegund sandfjara og mikilvægur afræningi á ungviði flatfiska, sér í lagi skarkola Pleuronectes platessa L. Markmið þessarar ritgerðar er að 1) kanna útbreiðslu og hugsanlega stækkun á útbreiðslu svæði sandrækju á Vestfjörðum. 2 ) skoða og meta hlutverk sandstranda á Vestfjörðum sem kjöruppeldisstöðvar fyrir skarkolann sem er mikilvæg nytjategund, en aflinn hefur dregist nokkuð saman undanfarin ár. Til að svara settum markmiðum var sýnum safnað dagana 24 – 31 júlí með bjálkatrolli á 11 sandströndum. Niðurstöðurnar benda til að útbreiðsla sandrækju nái nú að minnsta kosti norður í Bolungarvík. Hæsti þéttleiki sandrækju var á Brjánslæk (26.67 einstaklingar á 100m2), en nokkrar kynslóðir voru greinanlegar sem bendir til þess að stofninn er að fjölga sér. Hár þéttleiki af skarkola (hærri en 200 einstaklingar á 100m2) var ennfremur á þremur stöðvum. Niðurstöðurnar veita grunnupplýsingar um mögulega hættu sem skarkolaseiðum gætu stafað af sandrækju á þessum slóðum. Rannsóknin skilgreindi ennfremur áður óþekkt mikilvæg uppeldissvæði fyrir skarkola á Vestfjörðum sem hjálpar til við að vernda og nýta þennan mikilvæga nytjafisk.

Table of Contents

List of Figures ...... viii List of Tables ...... ix Acknowledgements ...... xi 1. Introduction ...... 1 1.1 Project aims ...... 3 1.2 Materials and Methods ...... 4 1.3 Limitations of the study ...... 4 1.4 Paper outline ...... 4 2. Theoretical overview ...... 7 2.1 Global invasive species trends ...... 7 2.2 Invasive species management ...... 8 2.3 Invasive species management plans ...... 9 2.4 Introduced species in Iceland ...... 10 2.5 Brown shrimp ...... 11 2.5.1 Ecogeography ...... 11 2.3.1 Reproduction ...... 12 2.3.2 Food web interactions ...... 13 2.3.3 Fisheries significance ...... 13 2.4 Icelandic Plaice ...... 13 2.4.1 Ecogeography ...... 13 3.4.1 Reproduction and spawning ...... 14 3.5 Influence of brown shrimp predation on flatfish ...... 16 4 Research methods ...... 17 3.1 Site description ...... 17 3.2 Oceanographic conditions ...... 17 3.3 MRI survey for P. platessa and C. crangon ...... 18 3.4 Sampling stations ...... 18 3.5 Sampling Techniques ...... 20 3.6 Lab Analysis ...... 20 3.7 Data Analysis ...... 21 4 Results ...... 23

4.1 Range expansion of brown shrimp ...... 23 4.2 Shrimp length distribution ...... 25 4.3 Plaice density and length distribution ...... 27 4.4 Shrimp density as a function of plaice density ...... 32 5 Discussion ...... 33 5.1 Range expansion of the brown shrimp...... 33 5.2 Plaice densities in the Westfjords ...... 36 5.3 Potential impacts of the C. crangon expansion in the Westfjords ...... 41 5.4 Management Recommendations ...... 42 5.4.1 Essential habitat designation ...... 42 5.4.2 Monitoring plan ...... 42 5.4.3 Vector management ...... 42 5.4.4 Areas of future research ...... 43 5.5 Conclusions ...... 44 References ...... 47 Appendix A ...... 54 Appendix B ...... 56

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List of Figures

Figure 1. Plaice harvest in Iceland from 1993-2011 ...... 3

Figure 2. Invasive species management framework proposed by Blackburn, et al. (2011)...... 9

Figure 3. Map of sampling stations in the Westfjords in 2012...... 19

Figure 4. Map of the relative density of shrimp found in the Westfjords...... 24

Figure 5. Distribution of brown shrimp in each size class...... 25

Figure 6. Kernel length density comparisons between shrimp across all sites in the Westfjords and the southwest station Álftanes...... 26

Figure 7. Map of relative densities of plaice in the Westfjords...... 27

Figure 8. Density of plaice at stations sampled in July of 2012 and 2006...... 28

Figure 9. Relative length distribution for plaice in the Westfjords of Iceland...... 29

Figure 10. Length distribution of plaice in the Westfjords of Iceland, from North to South. . 30

Figure 11. Relationship between plaice and shrimp densities...... 32

Figure 12. Map of nursery ground habitat for Icelandic plaice...... 40

viii

List of Tables

Table 1. Name and location of sampling stations ...... 19

Table 2. Shrimp density (100m-2) comparison between July 2006 and July 2012 in the Westfjords of Iceland...... 24

Table 3. Summary of July 2012 survey for plaice (Pleuronectes platessa) and brown shrimp (Crangon crangon) in the Westfjords of Iceland...... 31

Table B4. Length ranges for Icelandic plaice in the 1+ age-class caught at each station ...... 56

Acknowledgements

This project would not be possible without the guidance and assistance from those at the Marine Research Institute at Reykjavík and Ísafjörður. At the Reykjavík office, I would like to thank Björn Gunnarsson for the research design, historical data and field logistics and Ella Baranowska for taking time to show me the sampling protocol. At the Ísafjörður office, I am indebted to Jón Olafur, Hjalti Karlsson, and Maik Brotzmann for the generous donation of the lab vehicle, assistance with logistics and species identification, and letting me constantly fill the lab with the sweet aroma of decomposing fish. I am forever indebted to my trusty field assistant, my fiancé Evan Brunner, unwavering in his humor to spend countless hours collecting and measuring samples and the patience to show me programming language for my data analysis. Not only has his assistance made the scope of my data collection possible, but his continued support of this project has kept me always moving forward. I would also like to thank my father, Paul Koberstein, for some valuable perspective and proof-reading. Thank you to Dagný Arnarsdóttir for valuable feedback throughout the course of the project. Finally, I wish to thank my advisor at the Marine Research Institute, Jónas P. Jónasson, for helping me find the questions to ask, and answering them with new ones, from beginning to end. This project would not be possible without his valuable insight every step of the way.

xii

1. Introduction

Sandy shore ecosystems are integral for the development of juvenile flatfish species. These shallow shores serve as nursery grounds for young fish, protecting them from predation by larger fish while offering an abundance of food (Gibson, Robb, Wennhage, & Burrows, 2002). However, pollution, construction, and other habitat alteration activities can degrade coastal ecosystems (Brown & McLachlan, 2002). These threats to shallow, sandy shores pose a critical problem for coastal managers who must balance human activity and development with the ecological needs of our natural resources.

In Iceland, the majority of development, including residential, commercial and road construction, occurs along the coast. Construction can physically alter habitat, while pollution and nutrient-loading can enhance turbidity and algal growth, which decreases the quality of habitat for both juvenile flatfish and their prey (Brown and McLachlan, 2002).

Aquatic invasive species (AIS) can impact nearshore sandy bottom habitat by altering water quality via phytoplankton, altering substrate quality via algae or shellfish cover or through introducing predators with no strong top-down control in the new environment (Ruiz, Fofonoff, Carlton, Wonham, & Hines, 2000; Kostecki, Rochette, Girardin, Blanchard, Desroy, & Le Pape, 2011). Further, the impacts of aquatic invasions can be more difficult to detect than terrestrial ones (Wolfe, 2002). With AIS posing an ever-increasing threat to coastal diversity and habitat quality, the need for early detection and monitoring is critical to management of coastal ecosystems. Williams and Grosholz (2008) suggest that invasion science should receive a higher priority than has historically been given among coastal managers.

The Arctic has experienced relatively few invasions. However, changing climates may render environments that were previously hostile to new invaders more susceptible to successful invasions or render these habitats more hostile to native species (Canning- Clode, Fowler, Byers, Carlton, & Ruiz, 2011; Philippart, et al., 2011). Adequate monitoring is important to understand not only how community composition will change, but to increase the understanding of current species assemblages in less-studied regions.

The Conservation of Arctic Flora and Fauna (CAFF), a working group of the Arctic Council, emphasizes the importance of establishing baselines in Arctic regions in order to detect ecosystem shifts and utilize appropriate adaptive management strategies (Gill, et al., 2011). Establishing consistent and long-term monitoring regimes in Arctic environments is a priority for the Arctic Council (Gill, et al., 2011).

In 2003, the common or brown shrimp Crangon crangon L. was first recorded in Icelandic waters, though the time of colonization is estimated between 2001 and 2003 (Gunnarson, Ásgeirsson, & Ingolfsson, 2007). While incidental observations of the decapod date back to the late 19th century (Doflein, 1900), no established population has persisted until now. This current population is thought to have been introduced to Iceland via ship’s ballast water rather than climate-facilitated range expansion, as other populations already exist along similar climes (Gunnarson, et al., 2007). The primary concern for this recent colonizer is the potential impacts on juvenile plaice due to predation on nursery grounds. The brown shrimp co-occurs with 0-group plaice and is a significant factor in regulating the density of Icelandic plaice Pleuronectes platessa L. For example, van der Veer and Bergman (1987) observed density-dependant mortality among small (≤ 30 mm) plaice due to the predation of brown shrimp in the Dutch Wadden Sea. Wennhage (2002) suggests that this density-dependent mortality can result in reduced variation in plaice settlement density, both temporally and spatially. However, the effect of brown shrimp on 0-group plaice in Iceland remains uncertain. While established populations of the brown shrimp exist in the southwest and southern coasts, less is known about the relative density and distribution in the Westfjords region of Iceland.

Plaice is an important commercial flatfish species, both globally and in Iceland. Plaice have been commercially harvested in Iceland since the 19th century (Hjörleifsson & Pálsson, 2001). Between 1993 and 2001, annual harvest decreased by 61%, but has relatively stabilized around 6,000 tons since that time span (Figure 1). Despite the recent stabilization, the Marine Research Institute of Iceland (MRI) recommends a national quota limit of 6,500 tons for the 2012/2013 fishing season (MRI, 2011a). Globally, the Netherlands and Denmark are responsible for the highest landings of plaice, while Iceland represents approximately 6.5% of the total plaice harvest based on 2011 harvest numbers (FAO, 2012a).

14000

12000

10000

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6000 Catch (Tons) 4000

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Figure 1. Plaice harvest in Iceland from 1993-2011 (data gathered from Statistics Iceland, n.d.).

1.1 Project aims

This research project aims to develop a further understanding of the distribution of the brown shrimp in the Westfjords of Iceland and the extent of habitat overlap with native 0- group Icelandic plaice. The potential impacts of the brown shrimp expansion on 0-group plaice nursery grounds will be considered. Specifically, this project will answer three main questions: 1) What is the geographic extent of the European brown shrimp Crangon crangon in the Westfjords of Iceland? 2) What is the distribution and abundance of juvenile plaice nursery grounds, and what is the relative production of 0-group plaice in the Westfjords compared to other regions of Iceland? 3) What is the relationship between brown shrimp and juvenile plaice densities?

Densities of both Icelandic plaice and brown shrimp are sampled in sites throughout the Westfjords region and are mapped using the R environment (v.2.15.1). This will reveal density relationships as well as indicate previously unknown distributions of both plaice nursery grounds and brown shrimp range in the Westfjords. Because this study only takes place during one field season, its primary purpose is to establish geographic data on areas

3 of concern. Regions identified in this study as critical habitat can be used as baseline sites for continued monitoring programs. In addition, this information can facilitate best- practices for coastal development projects that aim to minimize environmental damage by mapping out areas of concern.

As Iceland is a country highly dependent on the strength of its commercial fisheries, understanding the dynamics of nursery ground habitat is crucial to maintaining commercially viable natural resources. In this study, the primary metric for measuring nursery condition will be production of plaice. While many other important factors should be considered concomitantly, this metric is a primary indication of habitat suitability (McConnaughey & Smith, 2000).

1.2 Materials and Methods

Brown shrimp and Icelandic plaice were collected using a 1 meter-wide beam-trawl with one tickler chain and 5.5 m long net, which was pulled through the water by two people at a constant speed. An average depth range of 0.5 to 1.5 m was maintained. Samples from the trawl were transferred into buckets and taken to the lab at the MRI in Ísafjörður for analysis. Data were analyzed using the R environment, and ArcGIS (v. 10.1) was used for mapping essential habitat.

1.3 Limitations of the study

Due to funding and time constraints, this project only collected samples during one summer field season. A longer dataset would be ideal, as this study can only compare results from the MRI survey in 2006 in the Westfjords and the southwest of Iceland. Therefore, conclusions regarding specific local densities of plaice are somewhat tenuous. Likewise, conclusions regarding the impact of the brown shrimp introduction and expansion are limited due to the scope of the project.

1.4 Paper outline

This paper first outlines invasive species trends in aquatic coastal ecosystems, and looks at the current status of AIS in Iceland. Management approaches are then considered. Then an in-depth look into the ecology of a recent invasive species, the brown shrimp Crangon

4 crangon, followed by a review of the ecology of native juvenile plaice and their nursery ground dynamics is given. This paper then outlines the methodology used to track the spread of the brown shrimp in the Westfjords region of Iceland as well as the significance of this region to juvenile flatfish productivity. The results of the field study are given, as well as a discussion of the findings and implications for coastal management.

2. Theoretical overview

2.1 Global invasive species trends

Human-mediated pathways accelerate biological invasions, and the impacts of such invasions have been well-documented for decades (Carlton, 1985; Pimentel, Lach, Zuniga, & Morrison, 2000; Ruiz, et al., 2000). Examples of human-mediated pathways include aquaculture and mariculture, ballast water exchange, hull fowling, fishing gear and bait, and intentional releases. In European coastal ecosystems, it has been suggested that 450- 600 species have been introduced via human-mediated pathways (Reise, Olenin, & Thieltges, 2006). With invasions comes cost, both economically and ecologically. For coastal environments, this is doubly true, as warming waters and busy waterway and port activity provide numerous vectors for introductions and dispersal (Carlton, 2010). Pimentel et al. (2000) estimated the annual cost associated with non-native species in the United States to be over US$130 billion. Regions with low species diversity and changing climates such as the Arctic and subarctic may be particularly susceptible to changes associated with species introductions (Philippart, et al., 2011).

Maritime transport has played a lead role in the rise in species introductions in recent decades (Carlton, 2010). Transport facilitates not only the spread of species via ballast water, fouling and other hitchhiking methods, but harbor infrastructure offers novel substrate to which species can attach (Carlton, 2010). On any given day, global shipping and transport vessels can harbor over 7000 species in their hulls (Tamlander, Riddering, Haag, & Matheickal, 2010).

Coastal ecosystems are among the habitats that experience the greatest impacts from invasive species. In some instances, native coastal marine communities have been almost fully replaced by invasive species, creating alternative states. For example, in the Gulf of Maine, a subtidal ecosystem shifted from one dominated by kelp beds of Laminaria spp. to a predominately invasive community composed of green algae Codium fragile tomentosoides, the brown algae Desmarestia aculeate, bryozoans, tunicates, and the

7 mussel Mytilus edulis (Harris & Tyrrell, 2001). The comb jellyfish was introduced to the Black and Azov seas in the 1980’s from ballast water originating in the western Atlantic coast. The introduction of this ctenophore was implicated in the severe decline of resident anchovy populations because the comb jelly population increased rapidly and out- competed other fishes for zooplankton (Kideys, 2002). In North America, the zebra mussel (Dreissena polymorpha) is unequivocally the most well-known example of AIS. This species has proven to be an aggressive invader, causing widespread monocultures, severe economic damage to aquatic infrastructure and disruptions throughout local food webs (Jaeger Miehls, Mason, Frank, Krause, Peacor, & Taylor, 2009).

Despite the breadth of literature on the impacts of AIS on coastal communities, it is important to examine the idiosyncrasies of each introduction to understand the dynamics at play. Such an examination would determine the scope and potential impacts as well as the possible ecological benefits an introduction could offer (Reise, et al., 2006). Effects of species introductions on the native community can be difficult to gauge. However, proxy metrics, such as changes in native community composition (species richness or evenness) may indicate effects (Reise, et al., 2006).

2.2 Invasive species management

Blackburn, et al. (2011) offer a comprehensive framework to assess biological invasions based on four stages of the invasion process: transport, introduction, establishment and spread (Figure 2). Once introduced, a species will either pass through the barriers illustrated towards successful establishment, or fail to overcome the barrier and thus fail to establish. Such failures to establish can be an artifact of species’ life history traits, environmental conditions or conditions of the receiving environment (i.e. level of biotic resistance) (Blackburn, et al., 2011). As a population arrives at each barrier, the framework offers useful definitions to describe the appropriate invasion status. For example, a population at the “D2” stage is defined as “self-sustaining population in the wild, with individuals surviving and reproducing a significant distance from the original point of introduction” whereas the “C1” stage represent “individuals surviving in the wild…in location where introduced” without reproducing (Blackburn, et al., 2011). Among the extensive literature on invasion science, a disparate set of definitions and terms

8 have emerged to describe similar processes, which may complicate the ability of those in invasion management to compare trends and issues (Valéry, Fritz, Lefeuvre, & Simberloff, 2008; Blackburn, et al., 2011). An advantage of this framework is that it standardizes the terminology and streamlines the theoretical framework. Another advantage is that it provides management a means to identify and analyze the extent of individual invasion events, by identifying intervention strategies.

Figure 2. Invasion framework proposed by Blackburn, et al., 2011. Alphanumeric codes represent unique definitions for species whose circumstances are described at each stage. Figure reprinted with permission from Elsevier.

2.3 Invasive species management plans Invasive species management plans are a means to establish protocol regarding invasive species. This includes, but is not limited to, vector management, problem species identification, response plans and delimiting funding/resources for management actions. In addition, management plans can encourage risk assessments for current/potential introduced species and establish monitoring protocols. The Global Invasive Species Program (GISP) has published a best practices toolkit for the prevention and management of invasive species (Wittenberg & Cock, 2001). This globally-applicable document

9 highlights the importance of a clearly delineated national vision, with goals and objectives that outline a national strategy on invasive species prevention and management.

Focused management plans addressing invasive species can provide ecological benefits as well as substantially large net economic benefits with successful preventative measures (Keller, Frang & Lodge, 2008). The State of Alaska, like Iceland, has relatively few invasive species compared to global levels, but has developed a comprehensive plan to manage current and future scenarios. This Aquatic Nonnative Species Management Plan addresses the control of vectors to prevent future introductions and establishes eradication measures for current high-risk and high-impact species (Fey, 2002). The plan also identifies numerous policy and knowledge gaps in terms of aquatic invasive species management. The plan identified the lack of an invasive species prevention program, and set out a plan to establish emergency response plans for high risk species and to develop a state-wide reporting system.

Iceland currently has no encompassing invasive species management plan in place. The Ministry of the Environment and Natural Resources and the Ministry of Industry and Innovations are responsible for invasive species control. Iceland’s primary strategies for the management of invasive species include its Strategy for Biological Diversity, drafted in 2008 with an implementation plan published in 2010, and provisions for invasive species provided in six acts and regulations, including regulations for ballast water enacted in 2010 (Hildur Vésteinsdóttir , Umhverfisstofnun, personal communication). In light of the growing list of aquatic species that have been introduced into Iceland’s coastal waters (NOBANIS, n.d.), the need for a comprehensive and current management plan should be given serious consideration.

2.4 Introduced species in Iceland Iceland has experienced fewer species introductions relative to global levels, but the number of introduced species is on the rise, according to NOBANIS (n.d.), the European Network on Invasive Alien Species, a project database for alien species in 17 European countries. Iceland hosts 7 invasive and 19 potentially invasive species (NOBANIS, n.d.). Terrestrial introductions have been easier to track compared to aquatic introductions, due to higher visibility. The seven invasive species in Iceland include the American mink

(Neovison vison), Alaska lupine (Lupinus nootkatensis), the moss Campylopus introflexus, cow parsley (Anthriscus sylvestris), a freshwater snail (Physella acuta), the Portuguese slug (Arion lusitanicus) and the white-tailed bumblebee (Bombus lucorum) (von Schmalensee, 2010). Horticulture is overwhelmingly implicated as the primary pathway for species introductions, but aquatic activities including ballast water exchange also present a measurable risk. Since around the beginning of the 21st century four new species have been discovered in Iceland’s coastal waters, the majority of which are suspected to have arrived via human-mediated pathways (Óskar S. Gíslason, Háskóli Íslands, 2012, personal communication). These introductions include the Atlantic rock crab, Cancer irroratus, the vase tunicate Ciona intestinalis, and the brown shrimp Crangon crangon (Óskar S. Gíslason, Háskóli Íslands, 2012, personal communication). The fourth species, European flounder Platichthys flesus, has most likely arrived through climate-induced range expansion. The implications of the brown shrimp introduction, in particular for juvenile plaice on nursery ground habitat, are given further consideration for the remainder of this paper.

The following sections describe the life history, distribution, ecological and economic significance of both C. crangon and the native Icelandic plaice Pleuronectes platessa.

2.5 Brown shrimp

2.5.1 Ecogeography

The brown shrimp is a member of the family Crangonidae under the order Decopoda. Brown shrimp occupy shallow, sandy bottom shores predominately at depths between 0 and 20 m (FAO, 2012). Early in the year, shrimp concentrate at depths greater than 1 m, and individuals migrate to shallower waters between 0.5 and 1 m by mid-summer (Gibson, et al., 2002). Globally, the brown shrimp experience a wide geographic range from temperate to subarctic latitudes at salinities between 5 to 35 psu (Campos, Moreira, Freitas, & van der Veer, 2012). The brown shrimp has historically occurred throughout Europe from the North Sea off of Norway in its northern limit to the Mediterranean as far south as Morocco, and also occurs in the White, Black, Baltic and Adriatic seas (Campos & van der Veer, 2008; Campos, et al., 2012). Luttikhuizen, Campos, van Bleijswijk, Peijnenburg, & van der Veer (2008) propose four major subpopulations based on phylogenetic spacing for

11 the brown shrimp: the northeast Atlantic, the Adriatic, the Black Sea and the Mediterranean. The brown shrimp found in Iceland are considered part of the northeast Atlantic cohort (Luttikhuizen, et al., 2008). However, further phylogenetic research is necessary to resolve subspecific as well as geographic boundaries for the brown shrimp and the wider genera (Campos, et al., 2012). Though brown shrimp tolerate a wide temperature range, they generally have a lower tolerance for environments with both low temperature and low salinity, most likely due to a depressed ability to osmo-regulate at lower temperatures (Lloyd & Yonge, 1947).

Throughout its range, the brown shrimp is a dominant species, representing up to 90% of the fish and macrocrustacean density in coastal, sandy bottom communities (Beyst, Hostens, & Mees, 2001; Campos, et al., 2012). Along the Belgium coast, densities up to 248.9 individuals per 100 m2 were measured (Beyst, et al., 2001). Along the French coast of the English Channel, average summer shrimp densities of up to 340 individuals per 100 m2 were found (Selleslagh & Amara, 2008). In Iceland, Gunnarsson et al. (2007) recorded shrimp along the west and south coasts, but found no shrimp along the north or east coasts. High densities have been observed in Iceland. In 2008, a mean density of 1573 individuals per 100 m2 was measured for the month of September (Guðmundsson, 2010). Shrimp were found in the Westfjords, but were limited to the southwest of the peninsula at a maximum density of 9.3 individuals per 100m2 (Gunnarsson, 2007).

2.3.1 Reproduction Brown shrimp are sexually dimorphic once they reach the age of maturation, and females generally have a higher growth rate than males (Lloyd & Yonge, 1947; Oh, et al., 1999). Sexual maturity is reached at approximately 30 – 45 mm in length for females and 22 – 43 mm in length for males (Kuipers & Dapper, 1984; Oh, Hartnoll, & Nash, 1999; Campos, et al., Veer, 2012); the size at maturation varies with location (Lloyd & Yonge, 1947).

Shrimp have two main spawning events per year, which take place in the winter and late summer. The winter spawning is considered to be the more significant spawning event, and is timed by environmental variables such as temperature and spring food availability (Kuipers & Dapper, 1984; Siegel, Damm, & Neudecker, 2008). Kuipers and Dapper (1984) summarized the reproductive strategies of the brown shrimp. Eggs produced during winter time are generally larger than those produced during the summer

12 spawning season. Females are ovigerous. Development time of eggs is correlated with temperature, but the range is approximately 3.5 to 13 weeks. The pelagic stage spans five weeks, after which shrimp join the benthos, at an average size of 4.7 mm. In the Irish Sea, Oh, et al. (1999) measured the onset of sexual maturity in shrimp to range from approximately 33 to 42 mm in length. The average life span of brown shrimp is approximately 3 years.

2.3.2 Food web interactions Brown shrimp play a key role in marine food webs. Brown shrimp are opportunistic feeders that concentrate peak feeding activity around dawn and dusk (Pihl & Rosenberg, 1984; Ansell, Comely, & Robb, 1999). While specific diet depends on prey availability, brown shrimp prey on other crustaceans, amphipods, filamentous algae, polychaetes, as well as flat and roundfishes (Ansell, et al., 1999; Wennhage, 2002). Size class and season impact prey choice, although mysids tend to represent large proportions of diet across size classes and seasons (Oh, Hartnoll, & Nash, 2001). In the seas of Sweden, brown shrimp consumed predominately ostracods and harpacticoids among the meiofauna, and macrofaunal species such as Nereis spp., bivalves and mysids (Pihl & Rosenberg, 1984). Brown shrimp are an important prey item for species such as Cancer maenas and 0-group plaice larger than ~30 mm in length. Gadoid species such as cod and whiting are significant predators of the brown shrimp, and can affect shrimp densities (Siegel et al., 2005).

2.3.3 Fisheries significance C. crangon represents a considerable fisheries market in many regions. In 2012, the global catch for the brown shrimp was 44,071 tons (FAO, 2012b). Historically, Germany accounted for the majority of brown shrimp landings, although in the past decade the Netherlands has equaled or surpassed Germany in tons landed (ICES, 2008).

2.4 Icelandic Plaice

2.4.1 Ecogeography Icelandic plaice (Pleuronectes platessa) is an ecologically and economically important fish species in Iceland. Plaice occur throughout Europe from the Mediterranean Sea to the

North Sea, along the northern limit of Europe to the White Sea, as well as in Iceland and occasional populations off of the coast of Greenland (FAO, 2012a).

3.4.1 Reproduction and spawning The primary spawning grounds for plaice in Iceland are located along the south and west coasts of Iceland, while secondary spawning locations exist throughout Iceland’s coastal region (Gunnarsson, Jonasson, & McAdam, 2010). Previous research had indicated that the primary spawning grounds for plaice occur in the southwest of Iceland, however recent efforts have determined that spawning occurs throughout Iceland’s coastal areas (Gunnarsson, et al., 2010). Breiðafjörður Bay on the west coast of Iceland has been identified as a significant spawning area (Solmundsson, Karlsson, & Palsson, 2003; Solmundsson, Palsson, & Karlsson, 2005). Analysis of 0-group otoliths post-settlement revealed that the majority of eggs in the Westfjords region were hatched in mid-April, with a mean larval period of 61 days (Gunnarsson, et al., 2010). Temperature has a strong influence on the length of the larval phase. The larval phase in the Westfjords is longer than that of the south, due to the difference in water temperature (Gunnarsson, et al., 2010). Plaice eggs and larvae are pelagic, and are carried to nursery areas along the coast via currents. Metamorphosis takes place around 8-10 weeks or approximately 13mm, after which larvae change from a laterally compressed conformation to a settled flatfish along sandy shores. Several factors affect juvenile plaice growth, including temperature, primary productivity and genetic adaptation (Hjörleifsson & Pálsson, 2001).

The larval phase is a significant stage for plaice population dynamics, as well as other flatfish species. Among dab, Limanda limanda, Beggs and Nash (2007) determined that the strength of the larval phase class can determine the level of recruitment for a population. This pattern was also observed among plaice (Nash & Geffen, 2012). Settlement density of 0-group plaice is significantly correlated with larval density (Wennhage, Pihl, & Stål, 2007). In Iceland, settlement begins in May, and nursery ground densities reach a maximum towards the end of June (Hjörleifsson & Pálsson, 2001). Plaice have a strong fidelity to both spawning and feeding grounds in Iceland (Solmundsson, et al., 2005).

Like the brown shrimp, plaice nursery grounds are located on shallow, soft bottom coastal regions. Post-settlement, 0-group plaice undergo numerous density-independent

14 and dependent forces. Patterns of density-dependent growth have been observed among larval plaice (van der Veer, 1986). Nash, Geffen, Burrows and Gibson (2007) observed a “self-thinning” pattern among juvenile plaice, whereas increases in the mean weight of the population exceeding carrying capacity led to subsequent decreases in density. Survival of 0-group plaice post-settlement is dependent on many factors, but temperature, food availability and predators can exert the strongest pressures (Gibson, 1994). Crangon crangon is a significant predator for plaice in most waters, as is the shore crab Cancer maenas and gadoid fish (van der Veer & Bergman, 1987; Ansell, et al., 1999; Freitas, Campos, Skreslet, & van der Veer, 2010). However, before the brown shrimp established in Icelandic waters, Hjörleifsson & Pálsson (2001), found little evidence of predation by crabs, but suggested birds and other fishes were most likely predators of 0-group plaice. Gibson (1994) infers that birds and other fish prey on plaice, and evidence of plaice have been found in the gut contents of arctic char. Cancer maenas is another common predator on juvenile flatfishes in many waters, but has been found at relatively low densities when sampling for plaice (Björn Gunnarsson, MRI, personal communication). In the Dutch Wadden Sea, summer predation by cormorants on 0-group plaice amounted to an estimated 12.55 million plaice, or 46% of all flatfish consumed by cormorants (Leopold, van Damme, & van der Veer, 1998). Cormorants and shags also prey heavily on plaice in Icelandic waters, consuming roughly 12 million individuals annually, mostly on age 2, 3, and 4-year fish (Lilliendahl & Solmundsson, 2006). Decreases in nursery ground density also occur as the summer season progresses. As individuals on nursery grounds grow in length, older, larger individuals migrate off the tidal flats into deeper water (Hjörleifsson & Pálsson, 2001).

Alteration of plaice nursery grounds by habitat destruction, vegetation or introduction of predators can concentrate 0-group plaice onto fewer ideal areas. Plaice tend to avoid areas of high algae cover and other substrate that limits their ability to burrow (Wennhage, 2002). An invasion by the limpet Crepidula fornicata significantly limited the burrowing capacity of juvenile plaice in the Mont-Saint Michel Bay in France, reducing the nursery grounds available for plaice in that bay (Kostecki, et al., 2011).

Previous studies have found that the Westfjords of Iceland represent one of the most significant nursery ground regions for plaice by density (Gunnarsson, et al., 2010). Densities measured in Iceland are also among the highest in Europe, although further

15 research can clarify the relative distribution of 0-group plaice densities across larger geographic regions (Gunnarsson, et al., 2010).

3.5 Influence of brown shrimp predation on flatfish

Predators can influence all life history stages of their prey in some respect. 0-group plaice will congregate in shallower waters to avoid predation, whereas larger individuals (>50) tend to be found at greater depths (Gibson, et al., 2002). The presence of brown shrimp can influence habitat selection by plaice, and cause plaice to avoid areas of predatory shrimp (Wennhage & Gibson, 1998). Shrimp are considered the primary predators on young plaice, and research has indicated that shrimp have a stabilizing effect on juvenile plaice densities (van der Veer & Bergman, 1987; Wennhage, 2002), but the current impact on plaice densities in Iceland remains uncertain (Gunnarsson, et al., 2010). van der Veer and Bergman (1987) found that only shrimp measuring over 30 mm in length have the potential to prey on plaice.

Along with predator density, abiotic factors can also affect mortality due to predation. Nakaya, Takatsu, Joh, Nakagami and Takahashi (2007) found that bottom water temperature was positively correlated with predation rate of the shrimp Crangon uritai on the marbled sole, Psuedopleuronectes yokohamae.

Another possible result of the brown shrimp introduction is prey competition. Gibson, Pihl, Burrows, Modin, Wennhage, & Nickell (1998) determined shrimp to be a primary competitor for prey on nursery grounds during the summer. Juvenile plaice feed primarily on benthic and demersal prey, namely polychaetes, bivalves and crustaceans, including the brown shrimp C. crangon (Beyst, Cattrijsse, & Mees, 1999). Brown shrimp and plaice show overlap in prey selection, including mysids and polychaetes. Therefore, the expansion of brown shrimp onto juvenile nursery grounds in the Westfjords may threaten plaice food availability, especially if 0-group plaice are not adapted to strong prey competition and environmental conditions weaken prey densities. However, this interaction may be neutral, as the presence of shrimp also provides a food source for larger plaice (Campos, et al., 2012). The competitive and predator-prey interactions on nursery grounds in Iceland remain unclear.

4 Research methods

3.1 Site description The Westfjords is the northwest peninsula of Iceland that spans 22,271 km2, representing 22% of Iceland’s total landmass. The basaltic bedrock was carved during the Pleistocene period, forming steep slopes up to a height of 700 m. The coastline is composed predominately of rocky outcrops and moderate to severely steep rocky shores, with relatively few sandy beaches suitable for juvenile plaice development. Iceland’s total coastline spans 6500 km in length; less than 10% (560 km) of which represents sandy, non- vegetated habitat (Gunnarsson, et al., 2010). The mean yearly air temperature and precipitation, which falls predominately as snow, in the Westfjords is 2.9°C and 969 mm, respectively (Decaulne, 2007).

3.2 Oceanographic conditions Icelandic’s surrounding waters are characterized by two main currents. The North Atlantic Drift brings warm (~10°C), saline (~35.2) water from the Atlantic Ocean. This flow creates the Irminger Current, which travels northwards along the west coast and turns into the North Icelandic Irminger Current once it turns east over the north coast of Iceland (Logemann & Harms, 2006). The other main current, called East Greenland Current, brings cold (less than 0°C) and fresher (34.5) water from the East Greenland Shelf (Jónsson, n.d.). Strong northerly winds can reduce the flow of Atlantic water up through Iceland (Astthorsson, Gislason, & Jonsson, 2007). In turn, the duration and degree of summer primary production is strongly influenced by the flow of the Atlantic drift (Astthorsson, et al., 2007). During the spring and summer, coastal regions experience a current of fresher water sourced from terrestrial runoff, which circulates clockwise around Iceland (Jónsson, n.d.). Iceland experiences semidiurnal tides, which start in the southwest and move clockwise around the island (Malmström, 1958). The average tidal range for the South and West of Iceland spans from 2 m during neap tide to 4 – 5 m during spring tide, with lower tidal ranges in the north and east (Malmström, 1958).

3.3 MRI survey for P. platessa and C. crangon

In 2006, the MRI surveyed 30 stations around the entire Iceland coast to sample the range and density of C. crangon in Icelandic coastal waters (Gunnarson, et al., 2007). Six of those stations were located in the Westfjords region. The authors found shrimp in only two of the six stations surveyed: Patreksfjörður and Brjánslækur. This project compares size and density data collected in 2012 with results found in the 2006 survey.

3.4 Sampling stations

In order to determine the geographic spread of C. crangon since 2006, as well as determine the density of juvenile plaice on nursery grounds in the Westfjords, sampling took place at eleven sites in the Westfjords region of northwest Iceland (Figure 3). Six sites were replicated from Westfjords stations sampled in Gunnarson, et al. (2007). These stations include Patreksfjörður, Dýrafjörður, Önundarfjörður, Bildudalur, Steingrímsfjörður, and Brjánslækur. An additional five sites were chosen based on beach characteristics (barren, sandy bottoms), access and adequate geographic spacing of stations. The northern-most station was Hesteyri, located on the Hornstrandir Nature Preserve. Only one station, Steingrímsfjörður, was sampled on the east coast of the Westfjords. The majority of stations were located in sites protected from strong wave action. A notable exception is Skálavik, which is a small bay exposed to the open ocean. A complete list of stations and GPS coordinates is given in Table 1. Sampling occurred within two hours of low tide, from July 24 – 31. Due to the northern latitude of the Westfjords region, the extended duration of daylight hours during the sampling period offered minimal variation of daylight present among stations. Distinct patterns of density differences by level of daylight have not been clear (van der Veer & Bergman, 1987), although evidence of diurnal and nocturnal density differences on tidal flats have been observed (Addison, Lawler, & Nicholson, 2003). All stations were sampled during the daytime with the exception of Brjánslækur, which was sampled at 23:35 hrs.

Table 1. List of sampling stations and their GPS coordinates, 2012 survey dates and number of replicate tows conducted at each station.

Station # of Number Station Name GPS Location Data Surveyed Tows 1 Bolungarvík 66°09.0180, 23°13.9710 24-Jul 3 2 Skálavík 66°10.9360, 23°28.6840 24-Jul 3 3 Dýrafjörður 65°53.9470, 23°29.5620 25-Jul 3 4 Önundarfjörður 66°01.6520, 23°26.0110 25-Jul 3 5 Hagakotsvík 66°00.6570, 22°38.3610 26-Jul 3 6 Steingrímsfjörður 65°46.2190, 21°42.6370 26-Jul 3 7 Súgandafjörður 66°08.3080, 23°31.3500 27-Jul 3 8 Hesteyri 66°19.9820, 22°52.3840 28-Jul 3 9 Brjánslækur 65°30.9470, 23°10.4090 30-Jul 3 10 Bildudalur 65°42.9580, 23°42.3440 31-Jul 3 11 Patreksfjörður 65°33.1060, 23°56.4440 31-Jul 3

Figure 3. Sampling locations in the Westfjords during July 24 – July 31, 2012. Numbers refer to station names given in Table l.

3.5 Sampling Techniques

Shrimp and plaice were collected using a 1 m wide beam trawl with a tickler chain. The trawl is 5.5 m long, with 8mm mesh size in the main body and 5.5 mm at the cod end. The trawl was pulled by two people maintaining a constant speed when possible. Three transects of 50 m in length were performed at each site, with each transect alternating direction. A handheld GPS unit was used to track distance of each tow. An average ocean depth of 0.5 – 1 m at each sample site was maintained, as this is the depth at which the highest density of 0-group plaice is found post-settlement (Gibson, et al., 2002). Contents of each tow were immediately transferred into buckets of seawater and taken to the lab for processing (see Appendix A for photos of field and lab work).

3.6 Lab Analysis

Up to 140 juvenile fish from each transect were identified and measured to the nearest millimeter in total length, and all plaice were counted and stored in 100 % ethanol. Shrimp were measured to the nearest millimeter from the tip of the rostrum to the end of the telson. Though brown shrimp display sexual dimorphism (Tiews, 1970), distinction by sex was only noted in the case of ovigerous females. Crab and other fish species collected in each tow were also identified and measured. An average of 50 individuals from each site were examined under a stereoscope to determine if any flounder Plactichthys flesus were present in the samples. Flounder have recently colonized Icelandic waters, and have been observed in regions close to plaice nursery grounds (O’Farrell, 2012). Differentiation between species was determined by first counting the number of caudal fin rays. Plaice commonly have 19-20 caudal fin rays, whereas flounder have 18. If an individual fish had fewer than 20 rays, the anal and dorsal fin rays were also counted. Dorsal and pectoral fin rays are more variable, and can be influenced by environmental conditions such as temperature and salinity (O’Neill, Keirse, McGrath, & Brophy, 2012). Average dorsal fin ray counts for flounder in the North Atlantic and Irish Seas range between 60 and 62, with an overall range of 54 to 68; flounder anal fin rays spanned an average of 42 – 44, with a range of 37 to 47 O’Neill, et al., 2012). For plaice, fin rays range from 57 – 80 for dorsal, and 43 – 61 for anal fins (Nichols, 1971).

3.7 Data Analysis Plaice sampled in 2012 were transformed to number of individuals per 100 m2 in order to standardize densities from previous years. Plaice length measurements were summarized by size classes to determine size distribution by station. All plaice measuring 70.0 mm or longer were excluded from the analysis of juvenile density, because plaice above this length are not considered young-of-the-year (Björn Gunnarsson, MRI, personal communication).

Density and length distribution data for plaice and shrimp from this project were compared to surveys conducted by the MRI in 2006 in the Westfjords. Results from the shrimp survey were also compared to data collected by collaboration between the MRI and the University of Iceland in the southwestern sampling station of Álftanes in Helguvík Bay to illustrate spatial differences in density and length. Comparisons of density and length distribution for plaice and shrimp were conducted using kernel density estimates in the R environment (v.2.15.1) (Tukey, 1977). For the 2012 plaice length distributions, data were smoothed with a GAM model using a cubic B-line smoother with 20 degrees of freedom (Venables and Ripley, 1997). Kernel density plots for shrimp length distribution data were created with the R base package. Data collected from 2012 were then mapped using ArcGIS software (v.10.1) to illustrate the special distribution of both species, as well as their relative densities.

4 Results

A total of 94 shrimp and 2812 plaice were collected between July 24th and 31st. Thirty plaice in the 1+ age-class (measuring greater than 70 mm in length) were caught, and excluded from the analysis (Appendix B). Other species collected on the tide flats include sand eels (Amodytes spp.), mysids, sculpin (Myoxocephalus scorpius), Norway pout (Trisopterus esmarki), European flounder (Platichthys flesus), the rock gunnel (Pholis gunnellus) and juvenile Atlantic rock crab (Cancer irroratus). C. irroratus was only found at Brjánslækur. High densities of sand eels were found at Steingrímsfjörður (n=135). No flounder were confirmed among the juvenile samples.

4.1 Range expansion of brown shrimp

Among the eleven stations sampled in this study, shrimp were present at six sites (Table 2; Figure 4). Four of these stations were also sampled in 2006. Shrimp were found at two additional stations not previously sampled. The northernmost station with brown shrimp present was Bolungarvík. No shrimp were found on the eastern station of Steingrímsfjörður.

The highest density of shrimp was found at Brjánslækur, with 26.67 shrimp per 100 m2. In total, eleven ovigerous females were observed (10 at Súgandafjörður and 1 at Brjánslækur). The average length for ovigerous females was 59.64 ± 5.89 mm. In Súgandafjörður, 71% of shrimp measuring greater than 50 mm in length were ovigerous females. At the same station, among shrimp measuring over 60 mm, the percentage increases to 83.

Shrimp densities in the Westfjords are on average lower than in the south of Iceland. The average density between June and August at sample station Álftanes in Helguvík Bay for 2005 and 2006 is 77.33 ± 43.0 and 58.33 ± 44.95 shrimp per 100 m2, respectively.

Table 2. Shrimp density (100m-2) comparison between July 2006 and July 2012 in the Westfjords of Iceland. 2006 data from Gunnarson, Ásgeirsson, & Ingolfsson (2007).

Station name 2006 2012 Bolungarvík 0 0.67 Dýrafjörður 0 0.00 Önundarfjörður 0 10.67 Steingrímsfjörður 0 0.00 Patreksfjörður 1 0.67 Brjánslækur 9.3 26.67 Súgandafjörður DNS 18.0 Bildudalur DNS 6.0 Hesteyri DNS 0.0 Skálavík DNS 0.0 Hagakotsvík DNS 0.0 N 94 DNS = Did not sample

Figure 4. Location and relative density of shrimp found in the Westfjords. Shrimp were found at six stations. The station with the highest abundance was Brjánslækur (26.67 ind. per 100m-2), the southernmost station surveyed.

4.2 Shrimp length distribution Mean length across all stations in the Westfjords was 35.37 ± (16.54), with a median length of 36.65. Brjánslækur has the highest abundance of juvenile shrimp (median length 13.65 mm, mean length 22.7 ± 14.16). No clear geographic gradient could be observed for length distribution, and low sample numbers prevented a clear analysis. The largest shrimp in terms of mean length were found in Súgandafjörður. In both Bolungarvík and Patreksfjörður, only one shrimp was found. The most abundant size class was between 36 – 45 mm (Figure 5). Overall, shrimp length displayed a bimodal distribution (Figure 6a), suggesting that new settlements are occurring in July. Mean shrimp length during a late- July sampling period also displayed a bimodal length distribution in the southwestern station of Álftanes in 2005 and 2006 (Figure 6b&c). Previous studies have confirmed that shrimp under 30mm in length are generally not large enough to prey on plaice of any size (van der Veer & Bergman, 1987). In this study, 69% of all shrimp caught were over 30 mm (n=66).

30 25 20 15 10 5 0 0 - 15 16 - 25 26 - 35 36 - 45 46 - 55 56 - 65 65 + Size class (mm)

Figure 5. Length frequency distribution of brown shrimp, all sites combined. All shrimp <25mm in length were found at Brjánslækur.

Figure 6. Kernel length density comparisons for shrimp across all sites in the Westfjords (a), and Álftanes in the southwest, in 2005 (b) and 2006 (c). Bandwidth varied based on sample size.

4.3 Plaice density and length distribution Plaice were present at all stations sampled (Figure 7; Table 3). Interannual and spatial variation was evident between 2006 and 2012 (Figure 8). Plaice density in Bolungarvík was nearly 80% lower than in 2006. Brjánslækur had a notably large increase in density from 2006.

Mean length across all stations was 29.02 ± 6.43, with the median length at 28.25. Plaice length distribution showed limited variation across stations, although larger plaice were abundant at Patreksfjörður, and the smallest plaice were found at the eastern station of Steingrímsfjörður (Figures 9 & 10). The most productive station in terms of plaice density was Dýrafjörður (504 plaice per 100m-2), followed by Brjánslækur and Hagakotsvík. Skálavik had the lowest density of plaice (8 plaice per 100m-2). All stations sampled in 2006, with the exception of Brjánslækur, had higher densities than in 2012.

Dýrafjörður Hagakotsvík

Brjánslækur

Figure 7. Map of relative densities of plaice in the Westfjords. Larger circles represent nursery areas with greater number of plaice present. The three highest density stations are highlighted.

1200.00

) 2 1000.00

800.00

600.00

400.00 2012 2006 200.00 Density (No. per 100m per (No. Density

0.00

Figure 8. Density of plaice at stations sampled in July of 2012 (grey) and 2006 (white). Error bars represent standard error. Five stations were not sampled in 2006: Bildudalur, Hesteyri, Hagakotsvík, Skálavík and Súgandafjörður.

Figure 9. Relative length (mm) distribution for plaice in the Westfjords of Iceland. Graphs are ordered by column from north to south to illustrate size structure gradient. Steingrímsfjörður is the sole eastern site. Solid black line is mean length distribution across all sample stations. Values are smoothed.

Figure 10. Length distribution of plaice in the Westfjords of Iceland, from North to South, arranged by columns. Steingrímsfjörður station is located in eastern Westfjords. Solid black line represents mean abundance across all sites.

Table 3. Summary of July 2012 survey for plaice (Pleuronectes platessa) and brown shrimp (Crangon crangon) in the Westfjords of Iceland. Mean length is given ±1 SD.

Total No. of plaice plaice Density (per Mean Length Min Length Max Length Number of Station counted measured 100m2) (mm) (mm) (mm) shrimp Bildudalur 108 105 72 ± 40.14 31.25 ± 6.17 20.05 68.95 9 Bolungarvík 67 60 44.67 ± 28.3 25.13 ± 4.61 15.5 37.84 1 Brjánslækur 592 419 394.67 ± 134.79 29.96 ± 3.95 18.7 49.55 40 Dýrafjörður 756 449 504 ± 61.29 25.45 ± 4.28 15.65 41.65 0 Hagakotsvík 568 450 378.67 ± 47.43 26.63 ± 4.37 12.9 39.95 0 Hesteyri 216 214 144 ± 51.89 29.14 ± 5.34 14.7 49.2 0 Önundarfjörður 35 29 23.33 ± 1.15 35.94 ± 10.85 19.35 60.2 16 Patreksfjörður 236 234 157.33 ± 38.7 37.35 ± 4.26 28.1 47.25 1 Skálavík 12 6 8 ± 2.0 33.90 ± 16.55 23.95 67.25 0 Steingrímsfjörður 68 68 45.33 ± 22.03 24.50 ± 7.75 12.2 45.5 0 Súgandafjörður 154 152 102.67 ± 60.87 31.49 ± 9.15 20.2 69.65 27 Total 2812 2186 29.02 ± 6.43 12.2 69.65 94

4.4 Shrimp density as a function of plaice density

The positive trend between shrimp and plaice densities on the six sites that contained both species was not significant (R²=0.4809, p>0.1; Figure 11). The trend is mainly driven by the high respective densities at Brjánslækur.

30.00

25.00

20.00

15.00

10.00 Shrimp of Density 5.00

0.00 0 100 200 300 400 500 Density of Plaice (ind. per 100m2)

Figure 11. Relationship between plaice and shrimp densities. The single site that contained high densities of plaice and shrimp was at Brjánslækur, and the majority of shrimp at this site were juveniles (<25mm in length). Two of the six sites had only one shrimp present, reducing the power of this analysis.

5 Discussion

5.1 Range expansion of the brown shrimp

This study has confirmed a northward expansion of the brown shrimp since 2006. The shrimp have expanded their range to the northern coast of the Westfjords as far as Bolungarvík. Fish larvae transport from southern Iceland northward have been facilitated by the clockwise-flowing Irminger current (Logemann & Harms, 2006; Gunnarsson, 2010), and this current could likewise facilitate the transport of shrimp larvae from subpopulations further south. Despite the increased number of shrimp measured in 2012 compared to 2006, densities in the Westfjords remain lower than those recorded in the southwest of Iceland. For example, shrimp density measured in Helguvík Bay in the southwest region of Iceland reached 185 individuals per 100 m2 in July of 2010 (Guðmundsson, 2010). Shrimp can display high interannual variability (Oh, et al., 1999), and with only two years for comparison, a distinct trend cannot be confirmed. However, it is interesting to note that while shrimp density in Patreksfjörður essentially remained the same in both 2006 and 2012, a large shift (640% increase) in shrimp density was observed at Brjánslækur in 2012. Most likely due to the brown shrimp’s wide native range, no previous studies have documented the shrimp as an invasive species.

Considering the presence of ovigerous females as far north as Súgandafjörður, spawning may be occuring in the Westfjords. Ovigerous females were only observed in the two stations with the highest corresponding shrimp density, Súgandafjörður and Brjánslækur, and only one shrimp in Brjánslækur was carrying eggs. Time of year and location influence the proportion of egg-carrying females (Lloyd & Yonge, 1947), so a longer dataset would be needed to illuminate spawning habits of shrimp at this latitude. A steep southeast to northwest temperature gradient in Iceland (Hanna, Jónsson, & Box, 2004) may produce regional differences in population structure, although this difference is minimal in July (Gunnarsson, et al., 2010). In southwestern Iceland, the percentage of females peaked in April at over 60%, and decreased to roughly 40% in July (Puro, 2010).

In addition, juveniles (< 20 mm) were observed at the lowest annual densies in May through July, but at the highest density in August (Puro, 2010). In this study, over half of the shrimp found at Brjánslækur measured less than 20 mm in length. It is unclear whether the juveniles observed at Brjánslækur originated from that site or if the larvae were transported from spawning locations farther south. This site, which was sampled on July 30th, may reflect a population structure more similar to the southwest population.

Ovigerous females predominated in size classes greater than 50 mm in length. This trend was also observed in the North Sea, where ovigerous females represented the majority of shrimp measuring over 50 mm in length (Siegel, et al., 2008).

In the northernmost station where shrimp were observed, only one shrimp was found. This could be due to a number of reasons. The day prior to sampling, strong winds and stormy weather may have caused both shrimp and plaice to seek shelter in deeper waters. Another possibility is that as the shrimp progresses northward, it may not have fully established at its northernmost range. Continued monitoring at this site will clarify population patterns.

Detectability of shrimp may be affected by diel activity (Addison, 2003). At greater depths, Addison (2003) caught a higher number of shrimp at night of the English coast. Off the Swedish coast, no significant difference between day and night abundance was observed for shrimp at shallow depths, but shrimp did tend to move inshore between evening and midnight (Gibson, et al., 1998). Conversely, the authors detected significant movement of plaice upshore from evening to midnight. The latest sampling time for this study occurred around midnight. The extended daylight hours in Iceland would most likely reduce diel variability in catch density.

Based on the Gunnarsson et al. (2007) hypothesis that ballast water exchange was responsible for introducing the brown shrimp to Iceland, between approximately 2001 and 2006 the brown shrimp would have most likely migrated against the current to the southeastern site of Hornafjörður where it was observed, if the point of introduction for the brown shrimp occurred at the port of Reykjavik. It is interesting that based on this research it took between six to eleven years for the brown shrimp to migrate north to Dýrafjörður with the advantage of the prevailing current, but only no more than approximately five years to travel against the current to the southeastern region of Hornafjörður. In

34 comparison, the Atlantic rock crab Cancer irroratus, which was first detected in Icelandic waters near Reykjavík in 2006, has been limited to the southwest and west coast, no farther north than Arnarfjörður in the Westfjords (Jόnas P. Jόnasson, MRI, personal communication). Although monitoring for this species has not yet been as prodigious as for the plaice and brown shrimp throughout Iceland, C. irroratus has not been detected along the south and east coasts. This leads to the question of whether other factors contributed to the presence of brown shrimp in Iceland. Such possibilities could include a second introduction at an eastern port such as Reydarfjörður, a port stop for the Icelandic shipping companies Eimskip and Samskip. However no shrimp have been detected near the port of Reydarfjörður, which reduces the likelihood of this scenario. Another possibility is the expansion of shrimp from nearby coastal regions such as the United Kingdom or the Faroe Islands. The Faroe Islands contain the port of Tórshavn, which serves as a connection between mainland Europe and Iceland. Ballast exchange in this region could introduce brown shrimp larvae from other regions in Europe, and currents from the Faroe Islands could carry larvae to southeast Iceland (Logemann, n.d.). Evidence of brown shrimp presence in the Faroe Islands is lacking, however.

Warming waters in Iceland could also facilitate the spread of the brown shrimp to its southern coastline from outside regions if the larvae would survive the trip. Though cycles of warming and cooling have occurred throughout the past century, Iceland has experienced an overall warming of approximately 0.8 to 1.6°C between 1878 to 2003 (Hanna, Jónsson, & Box, 2006). Sea bottom temperatures off the south coast have increased approximately 0.5°C between 1970 and 2011 (MRI, 2011b). Average monthly temperatures measured for 1991 to 2004 in the southwest of Iceland range between 1.3°C in March to 9.0°C in August, and a similar range was observed on the southeast coast (Hanna, Jónsson, Ólafsson, & Valdimarsson, 2006). Considering that shrimp are commonly found off the coast of Norway at temperatures from approximate 5 – 12°C (Campos, Freitas, Pedrosa, Guillot & van der Veer, 2009), Iceland’s southern climate should not necessarily present a barrier to shrimp expansion, as discussed in Gunnarsson, et al. (2007), though it may influence invasion success.

5.2 Plaice densities in the Westfjords

This study has identified high density nursery habitat for 0-group plaice in the Westfjords. At least three sites contained plaice densities greater than 200 individuals per 100 m2. In 2006, a July survey of juvenile plaice throughout Iceland’s coast indicated that the greatest density of Icelandic plaice was located in the Westfjords (Gunnarsson, et al.,2010). Among the nine sites out of 31 in 2006 that contained plaice densities greater than 200 individuals per 100 m2, three were located in the Westfjords (Gunnarsson, et al., 2010). This study found that the station at Dýrafjörður continues to have the highest density of 0- group plaice in the Westfjords. Patreksfjörður has also been previously identified as an area of high juvenile plaice production (Gunnarsson, et al., 2010). Along with these locations, new areas of high juvenile plaice density have emerged from this study, including Hagakotsvík, located deep in Ísafjarðardjúp, and Hesteyri, located along the Hornstrandir peninsula. Given the high density of juvenile plaice found at these sites, they should be considered critical habitat for juvenile plaice development. However, it is important to note that high density nursery grounds may shift. In 2006, Dýrafjörður, Bolungarvík and Patreksfjörður all contained plaice densities greater than 200 individuals per 100 m2 (Gunnarsson, et al., 2010). This dynamism suggests that management of nursery grounds should focus on the ideal habitat type at least as much as specific locations.

Size density distributions across all sites generally exhibited little variance from the mean. The two exceptions are Patreksfjörður, which exhibited a higher mean length, and Dýrafjörður, which exhibited a lower mean length. At such a small geographic scale, little variance among sites is expected. Gunnarsson, et al. (2010), observed higher growth rates in the warmer waters of the south and western Icelandic coasts, whereas lower growth rates were observed in the colder Westfjords region. If the larger size structure observed at Patreksfjörður is attributed to a temperature gradient, a similar size structure would be expected at Brjánslækur, which sits at nearly the same latitude. However, Brjánslækur had a smaller size structure, which fell closer to the mean. Therefore, a possible explanation is that timing of settlement may not be synchronous throughout the Westfjords. If larvae settled at Patreksfjörður earlier than other regions in the Westfjords, we would expect a larger size structure. Breiðafjörður is a primary spawning ground for plaice on the west coast (Solmundsson, et al., 2005). It is possible that larvae from the earlier spawning

36 populations in the southwest such as southern Faxafloi and Reykjanes (Solmundsson, et al., 2005) dispersed to the Westfjords, carried by the prevailing currents and contributed to early settlers in Patreksfjörður. The extent of the geographic dispersal in larvae may be impacted by factors such as the location of larvae in the water column and the timing of the spawning event (Vikebø, Sundby, Ådlandsvik & Fiksen, 2005). Climate change could lead to altered dispersal patterns, although some scenarios produce disparate effects. Biophysical models predict that warmer temperatures would reduce dispersal, while other affects such as increased freshwater runoff to coastal areas would increase dispersal (reviewed in Lett, Ayata, Huret, & Irisson, 2010). The smaller size structure observed in Dýrafjörður could also be due to late settlement by a different spawning group. Local environmental conditions, such as food availability, could also influence growth rate variance, although Hjörleifsson & Pálsson (2001) suggest that food is not a limiting factor among Icelandic plaice nursery grounds. Otolith readings, which are underway at the MRI lab in Reykjavik on these samples, would reveal specific ages of each subpopulation and possibly clarify this incongruity.

In Ireland, shrimp density has been positively correlated with juvenile plaice density during summer density surveys (De Raedemaecker, et al., 2012). However, Pihl (1990) observed an inverse trend between plaice density and the combined dry-weight biomass of two predators (C. crangon and Carcinus maenas) on the Swedish west coast. The authors indicate that both predator species were dominant at the site of this relationship, and did not include densities of each predator. In this study no meaningful relationship was determined between plaice and shrimp densities. This could be largely attributed to the low number of samples with shrimp present compounded with the fact that only four sites had more than one shrimp. While Önundarfjörður had the third highest density of shrimp among sites samples, it also contained the second-lowest density of plaice. A higher density of plaice was expected at Önundarfjörður based on past survey data and high habitat suitability. A possible explanation could be due to the sampling location. A strong current made the tow speed slower than average, which may have affected the quality of the data collection and increased the ability of species to avoid the sampling gear. Shrimp were also notably absent from some of the most productive nursery areas sampled, namely Dýrafjörður and Hesteyri. The absence at Dýrafjörður was curious, considering shrimp were present in the fjord immediately north (Önundarfjörður) and south

(Arnarfjörður at Bildudalur). Despite the positive association between plaice and shrimp densities found by De Raedemaecker, et al. (2012), the authors determined that shrimp density did not have a strong effect on the variability of plaice density. While the authors concluded that a low percentage of plaice density variation could be explained by habitat characteristics, depth and macroalgal cover were the strongest factors explaining variability between nursery grounds.

Low densities of plaice at Skálavik could be an artifact of environmental conditions. Skálavik is an exposed beach that receives heavy wave action, and though areas of the intertidal were free of algae, high algal cover along the beach was observed. Field and lab observations have indicated large reductions in plaice density in the presence of algae, as high as 60 – 90% (Wennhage, 2002; Pihl, et al., 2005; Troell, Pihl, Rönnbäck, Wennhage, Söderqvist, & Kautsky, 2005; De Raedemaecker, et al., 2012). Algae may concentrate young plaice in smaller patches, and increase density-dependent mortality due to predation by brown shrimp (Wennhage, 2002). Patches of algae were observed at Hagakotsvík and Brjánslækur, and mussel hummocks were also observed at Brjánslækur.

Plaice densities measured in this study are an order of magnitude greater than other nursery areas throughout its range. Densities of 0-group plaice in Galway Bay, on the west coast of Ireland never exceeded 29.6 individuals per 100m2 between 2008 and 2009 (De Raedemaecker et al., 2012). On the west coast of Scotland, July densities between 1995 and 1997 ranged from 5 to 20 individuals per 100m2, but increased to approximately 70 in August (Gibson, et al., 2002). Selleslaugh and Amara (2008) measured summer density (June through August) in 2000, 2005 and 2006 along the French coast of the English Channel and found an overall average of 9.1 individuals per 100m2, although spring densities were higher, at 49.9 individuals per 100m2. Long-term data in the Balgzand intertidal region of the Dutch Wadden Sea showed that plaice density rarely exceeded 50 individuals per 100m2 (van der Veer, et al., 2011). Densities comparable to Iceland have been documented in the Swedish Skagerrak archipelago, ranging from 200 to 1947 individuals per 100m2 in 1998; a drop trap was used to collect samples in this study, so direct comparisons are difficult (Pihl & Wennhage, 2000). While this study did not measure overall output of plaice, it is evident that in addition to the productive nursery of Faxafloi in the southwest, the Westfjords provides important nursery grounds for the Icelandic plaice (Hjörleifsson & Pálsson, 2001).

Critical habitat mapping is an important management tool to identify key habitats throughout a species’ life history (Valavanis, et al., 2008) and is a key step into determining the risk of a shrimp range expansion into the Westfjords. By examining aerial photography of the Westfjords, it is evident that sandy beach areas are limited in the Westfjords. The majority of coastline surveyed was characterized by steep, rocky shores, and algal cover was common. While this project could only sample 11 stations, at least 13 sites in the Westfjords display environmental characteristics indicative of nursery ground habitat, excluding the sampled site of Skálavík, which contained very low densities of plaice (Figure 12). These observations suggest that plaice nursery grounds are limited in the Westfjords. Consequences of limited nursery ground habitat include bottlenecks if spawning stock exceeds the carrying capacity of nursery areas (Pihl, et al., 2005). Further, the size of nursery ground habitat may affect recruitment (van der Veer, Berghahn, Miller, & Rijnsdorp, 2000; Pihl, et al., 2005). The limited quantity, or patchiness of essential nursery habitat in the Westfjords creates a distinct risk if any of the highly productive areas experience degraded quality or significant disturbance (Gibson, 1994).

Figure 12. Nursery ground habitat for Icelandic plaice. Open circles represent sites not sampled as part of this study, but contain characteristics similar to existing nursery habitat. Hashed circles represent sites sampled in this study. Red circles indicate sites with densities of brown shrimp greater than 10 ind. per 100m2. The X on the site of Skálavík is due to the low density of plaice (8 ind. per 100m2) found there, which suggests that this site is not an ideal nursery habitat site.

5.3 Potential impacts of the C. crangon expansion in the Westfjords

The presence of predators can impact nursery habitat quality of plaice through forces such as density-dependent predation (van der Veer & Bergman, 1987), size-selective predation, as small juvenile fish are more vulnerable to predation (Gibson, 1994), and by potentially stabilizing the density of juvenile plaice through time (Wennhage H. , 2002). The risk to 0-group plaice increases with the introduction of non-native predators if biotic resistance towards non-native species in Iceland’s coastal community is generally low (see future research recommendations). Biotic resistance refers to the predation pressures exerted by native prey on nonnative species, and can be influenced by the propagule pressure of the invader, its reproductive rate, the abundance of native predators and the functional response between the native predator and nonnative prey (Twardochleb, Novak, & Moore, 2012). Alternatively, successful invasions often have the advantage of “escaping” predation in their new environment (Wolfe, 2002). In the absence of a strong predator, an increase of shrimp posses an increasing risk to the limited essential habitat of the juvenile Icelandic plaice. Further information on the potential biotic resistance to brown shrimp would shed greater light on the overall risk to young plaice.

Once established, warming waters could improve the success of the brown shrimp in Iceland. Brown shrimp generally exhibit a wide tolerance for temperature (Beyst, et al., 2001), and population density is positively correlated with sea water temperature (Oh, et al., 1999). Temperature tolerances, along with salinity, greatly influence the densities of tidal fish species as well (Selleslagh & Amara, 2008). For plaice, van der Veer and Witte (1998) found that year-class strength decreases in years of warmer winter sea temperatures in the North Sea, the species southern range. In the northern limit of plaice, however, low temperatures generally delay the timing of the spawning season and duration of the growing season (Freitas et al., 2010), and cooler temperatures in late summer and early fall are linked to emigration by plaice off of nursery grounds (Hjörleifsson & Pálsson, 2001). Freitas et al. (2010) also suggest that conditions other than temperature may have stronger influences on local growth rates of plaice, including hydrodynamic features and food availability.

5.4 Management Recommendations

5.4.1 Essential habitat designation In light of the expansion of a new predator onto juvenile plaice nursery grounds in the Westfjords, the health of this ecosystem should be a primary concern in the management of the plaice fishery. Resistance to biological invasions may be lower in regions with relatively lower species richness such as Iceland, suggesting vulnerability in this region (von Schmalensee, 2010). This study has observed high-density nursery grounds in the Westfjords that may be valuable for the health of the plaice fishery. These grounds should be considered critical habitat in light of any future development or construction projects. The exception is Hesteyri, which is located on a nature reserve and thus is an already- protected habitat.

5.4.2 Monitoring plan Given the recent increase in species introductions to Iceland and the observed range expansion of the primary flatfish predator C. crangon, an invasive species management plan should be considered for Iceland. A management plan could be a valuable tool to identify essential fish habitat as sites of concern in the event of introduced species. Such a plan could instate a periodic monitoring plan to detect the population dynamics and distribution of the brown shrimp in the Westfjords. Regular sampling for the brown shrimp already occurs through the MRI in southwestern Iceland, where significant plaice nursery grounds are located. The sampling stations established in this project could serve as key stations for continual monitoring of shrimp and plaice densities in the Westfjords. While monitoring projects can be costly and involve high levels of manpower, Iceland could coordinate efforts with the Conservation for Arctic Flora and Fauna branch of the Arctic Council to enhance a knowledge base of Arctic and sub-Arctic biodiversity trends. Further, a monitoring plan could utilize citizen scientists for data collection, or at the very least create a self-reporting system for field observations of species of concern. Citizen science programs have been shown to increase social capital while increasing science literacy in communities (Conrad & Hilchey, 2011).

5.4.3 Vector management Management of the brown shrimp population in Iceland may be difficult once it is established on critical plaice nursery grounds. Therefore, the onus should rest on vector

42 management in Iceland (Reise, et al., 2006). Globally, ships’ ballast water transports an estimated 12 billion tons of water every year (Bax, Williamson, Aguero, Gonzalez, & Geeves, 2003). As an island nation, ballast water management presents a significant vector for the transport of AIS to Iceland, and this is likely to increase with the projected rise in shipping traffic (Protection of the Arctic Marine Environment, 2009). Iceland has identified ballast water management as a priority in its outline of government priorities for sustainable actions for the years of 2006 through 2009 (Ministry for the Environment and Nature, 2002).

The passing of the International Maritime Organization’s International Convention for the Control and Management of Ships' Ballast Water and Sediments will introduce further restrictions meant to curtail the spread of AIS, including a mandatory distance from shore to conduct ballast water exchange and ballast treatment standards. In order for the convention to enter into force, it must be ratified by at least 30 states, representing at least 35% of the global shipping tonnage (International Maritime Organization, 2011). As of January, 2013, 36 states have ratified the convention, but these states only represent 29% of the gross shipping fleet tonnage (International Maritime Organization, 2013). Therefore, standards set forth by the ballast water convention remain voluntary. Iceland has not ratified the convention at this time.

5.4.4 Areas of future research Brown shrimp exist in relatively low densities in the Westfjords, which may be an artifact of abiotic factors rather than biotic resistance. However, future research should investigate the addition of brown shrimp as a prey item in the marine food webs of Iceland. This could be achieved by determining the relative abundance of brown shrimp in the gut contents of two prominent predators, cod and whiting (Siegel, et al., 2008). A gut content analysis for the brown shrimp would clarify the prey utilization of this species in its new environment. This could shed light on the degree of prey overlap, if any, with the Icelandic plaice, and thus the level of competition this new species may introduce.

Invasive populations that maintain low densities experience genetic drift from bottlenecks. The resulting low genetic diversity may influence the success of the population (Hoelzel, Fleischer, Campagna, Le Boeuf, & Alvord, 2002). Future research on

43 the genetic structure of the Westfjords population could illuminate whether the Westfjords is a distinct subpopulation and if mixing occurs with populations in the southwest.

The patchiness of shrimp distribution in the Westfjords indicates that shrimp are still limited by biotic or abiotic factors. Local conditions, such as food availability and temperature may affect distribution. It is possible that the climate in this region is still limiting the proliferation of the species, as temperatures recorded in the winter months in the Westfjords are lower than those in the southwest. Temperature anomalies tend to be greater in the North of Iceland compared to the south (Hanna, et al., 2004), and stochastic events such as extreme cold could limit the survival of non-native species (Canning-Clode, et al., 2011). However, air temperatures in Iceland as a whole have increased on average based on long-term readings (Hanna, et al., 2004), and as time progresses, populations will most likely become more established and expansive. Further research should establish longer-term population trends for this species in the Westfjords in relation to temperature gradients and variances.

5.5 Conclusions

By mapping out the relative densities and distribution of the brown shrimp in the Westfjords, this project has determined a northward expansion of brown shrimp in Iceland. The shrimp are now found as far north as Bolungarvík, but remain almost entirely on the south and western coasts of the Westfjords. Population densities remain low in the Westfjords, however, and only one site (Brjánslækur) indicated new generations present. High densities of plaice (greater than 200 individuals per 100 m2) were detected in three sampling stations, and juveniles were detected at all stations, indicating that the Westfjords plays a key role in a pivotal life history stage of this species. Plaice densities measured in the Westfjords are among the highest reported throughout its range. This project also contributed to the baseline mapping of essential juvenile plaice habitat in the Westfjords, and illustrates the dynamism of nursery habitat densities. Taken together, this information highlights the threats posed to commercial fisheries at early life-history stages and the importance of the habitat that supports them. While it remains unclear whether brown shrimp have been introduced into Iceland, we can conclude that this population is expanding northwards onto essential habitat that has lacked a highly-abundant predator.

Changing climates create many uncertainties regarding coastal communities, including the impact of aquatic invasive species (Stachowicz, Terwin, Whitlatch, & Osman, 2002). What happens below the surface of our seas may be difficult to detect. A robust monitoring plan could not only enhance our understanding of these dynamics, but could simultaneously increase scientific engagement in coastal communities (Simberloff, et al., 2012). And finally, the increased spatial knowledge of commercially-important juvenile flatfish can enhance the best practices of coastal management to prevent degradation of these important habitats.

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Appendix A – Photos from the field Holding a juvenile plaice. Samples were transferred into buckets, and then taken back to the lab at the MRI in Ísafjörður for analysis. Photo credit: Clasina Jensen

Hagakotsvík, located deep in Ísafjarðardjúp. This is an example of a high-density nursery ground. Photo credit: Marla Koberstein

Ready to deploy the beam trawl. Photo credit: Clasina Jensen

European brown shrimp (above) and Icelandic plaice (right) were counted and measured at the MRI lab in Ísafjörður. Below: trays were used to sift through samples for plaice, shrimp and other fish species. Photo credit: Marla Koberstein

Appendix B

Icelandic plaice sampled in the 1+ age class were not included in the primary analysis.

Table A4. Length ranges for Icelandic plaice in the 1+ age-class caught at each station.

Length Site Min (mm) Max (mm) n Bildudalur 106.15 133 3 Bolungarvík 82.95 167.5 7 Brjánslækur 86.35 86.35 1 Dýrafjörður 225.75 225.75 1 Hagakotsvík 120.4 190.45 2 Hesteyri 75.3 119.35 6 Önundarfjörður 73.65 86.8 2 Patreksfjörður 74.8 99.1 6 Súgandafjörður 80.7 95.15 2 Skálavík 74.8 99.1 6 All Sites 73.65 225.75 30