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Sharks in Nearshore Environments: Models, Importance, and Consequences

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Sharks in Nearshore Environments: Models, Importance, and Consequences Vol. 402: 1–11, 2010 MARINE ECOLOGY PROGRESS SERIES Published March 8 doi: 10.3354/meps08498 Mar Ecol Prog Ser OPENPEN ACCESSCCESS FEATURE ARTICLE Sharks in nearshore environments: models, importance, and consequences Danielle M. Knip1,*, Michelle R. Heupel2, Colin A. Simpfendorfer1 1Fishing and Fisheries Research Centre, School of Earth and Environmental Sciences, James Cook University, Townsville, Queensland, 4811, Australia 2School of Earth and Environmental Sciences, James Cook University, Townsville, Queensland, 4811, Australia ABSTRACT: Theoretical models of coastal shark popu- lations have remained largely unchanged since the 1960s despite limitations in applicability to many spe- cies. Smaller bodied coastal species are poorly repre- sented by the current models. A new theoretical model is proposed to represent those species that spend most or all of their life within nearshore waters but do not show use of discrete nursery areas. Description of this new model outlines the importance of nearshore areas to these smaller species. While all coastal shark popu- lations are susceptible to environmental and anthro- pogenic impacts, species that fit the new model are more vulnerable to varying coastal processes, habitat degradation, and fishing pressure than are species that use nearshore areas for only part of their life-span. The dynamic nature of nearshore areas and their proximity to human populations present all sharks that occur in Population models show that nearshore habitats are crucial them with a range of advantages and disadvantages. for small coastal sharks such as juvenile lemon sharks This paper reviews how different species utilise near- Negaprion brevirostris. shore areas and how they overcome the challenges Photo: Alastair Harry they face in inhabiting these areas. Improving and ex- panding theoretical models of coastal shark popula- tions will provide a better understanding of how sharks use nearshore environments and assist in making con- Morin et al. 1992). Major fluctuations in salinity, tem- servation and management decisions for these regions. perature, depth, flow, and turbidity occur in nearshore waters on a variety of temporal scales (from hours to KEY WORDS: Nearshore · Shark · Population model · seasons) due to changes in tidal level, freshwater flow, Habitat use · Distribution · Springer rainfall, and seasonal weather patterns (Mann 2000, Resale or republication not permitted without written consent of the publisher Masselink et al. 2008). Although variable conditions may create challenging environments for inhabitants, nearshore areas are highly productive and have a INTRODUCTION relatively high abundance and a rich diversity of fish and invertebrate species (Blaber et al. 1989, Beck et Nearshore areas typically consist of shallow water al. 2001). As a result of high productivity, nearshore with temporally varying characteristics and are com- waters have considerable economic value supporting monly comprised of highly dynamic ecosystems sup- recreational, commercial, and indigenous fisheries. porting high biodiversity (Robertson & Duke 1987, Overall, nearshore areas contribute goods and services *Email: [email protected] © Inter-Research 2010 · www.int-res.com 2 Mar Ecol Prog Ser 402: 1–11, 2010 of high quantity and quality to both environment DeAngelis et al. 2008). Thus, habitat use by sharks and economy, resulting in these areas being identified within nearshore areas is likely to be influenced by a as significantly valuable ecosystems (Costanza et al. combination of ecological factors including environ- 1997). mental characteristics, resource abundance and dis- Close proximity to land allows easy accessibility to tribution, and/or presence of other competing species. nearshore areas, resulting in these regions being sus- Common nearshore shark species consist primarily ceptible to increased exploitation. Fishing is one of the of carcharhinids and sphyrnids in tropical and sub- major human impacts affecting nearshore waters and tropical regions, and triakids in temperate regions overfishing has resulted in the decline and/or collapse (Bigelow & Schroeder 1948, Compagno 1984, Last & of some coastal ecosystems (Pauly et al. 1998, Jackson Stevens 2009). Species from these groups tend to be et al. 2001). Coastal development also has detrimental well studied because they are encountered often in effects on nearshore areas through modification prac- coastal regions and are easily captured. These groups tices such as dredging, construction, and deforestation, also form the basis of some important fisheries, which which can cause large-scale habitat degradation or has further driven research (Grace & Henwood 1997, loss (Suchanek 1994, Vitousek et al. 1997). The esti- Francis 1998, Walker 1998, Pradervand et al. 2007). mated proportion of the world’s total human popula- However, the diversity of sharks that occur in near- tion living within 100 km of the coast is 60% (Vitousek shore waters is much greater than these 3 families et al. 1997), with this value projected to be 75% within alone (e.g. scyliorhinids, orectolobids, ginglymostom- 60 km of the coast by the year 2020 (DeMaster et al. atids, heterodontids). It is important to recognise the 2001). Due to increasing human population in coastal diversity of sharks within nearshore areas because dif- areas, it is likely that human pressure in these regions ferent species behave in different ways (Bethea et al. will continue and potentially increase. With increased 2004), have different life-histories (Cortés 2000) and, pressure, it will be progressively more important to as top predators, may have a large influence on near- understand how species and communities use near- shore community dynamics (Heithaus et al. 2008). shore waters so that effective conservation and man- Although the species composition of sharks that agement can be implemented. In addition to anthro- occur in nearshore waters is diverse, descriptions of pogenic influences, environmental impacts such as distribution and habitat use tend to be generalised. For weather events result in erosion, scouring, habitat example, a theoretical model proposed by Springer destruction, sediment movement, and increased tur- (1967) broadly outlined the geographic range and dis- bidity in nearshore areas (Rodriguez et al. 1994, Mas- tribution of a hypothetical population of sharks. In this selink et al. 2008). Inter-annual climate anomalies (e.g. population, young are born in nearshore nursery areas El Niño, La Niña, drought) can change the physical in spring/summer, where they remain until they reach characteristics of nearshore environments making con- sexual maturity and join the adult population further ditions less favourable for inhabitants (Mol et al. 2000, offshore. Adults occur offshore, segregated from the Abel et al. 2007). Since nearshore areas are highly young except when they move inshore to give birth dynamic and variable, as well as vulnerable to ex- and mate in spring/summer. However, not all popula- ploitation, species that inhabit these waters must either tions of sharks that occur in nearshore areas fit this cope with the changes they face, adapt accordingly, or model. The strategy a species utilises is shaped by both leave in order to survive. its life-history characteristics (Branstetter 1990) and Sharks are a key component of nearshore ecosys- surrounding environment (Sims 2003) to maximise sur- tems, acting as top predators and utilising a high pro- vival, which results in distribution and habitat use portion of available energy (Cortés 1999). Since near- varying greatly between species. Opposed to Springer’s shore waters provide a wide variety of habitat hypothetical population, a combination of life-stages characteristics, sharks can exploit regions with fea- may be present in nearshore regions, with species tures that are optimal for survival. For example, young using patterns that will enhance population success. sharks may utilise shallow or low salinity environments Understanding differences in distribution and habi- to decrease predation risk (Simpfendorfer et al. 2005, tat use between shark species that utilise nearshore Wetherbee et al. 2007) or to forage in areas where food areas will help effectively conserve important habitats resources are most abundant (Simpfendorfer & Mil- and the populations that use them. This review paper ward 1993); adults may exploit habitats to target high outlines and discusses (1) theoretical models of shark quality prey items for diet and growth (Heithaus et al. populations in nearshore areas, (2) how sharks utilise 2002). Nearshore regions are also used in different nearshore areas, and (3) challenges and potential con- ways by different shark species, and the characteris- sequences sharks face by inhabiting nearshore waters. tics and habitat type of a region may influence species By discussing how shark species use nearshore regions distribution and movement (White & Potter 2004, and identifying differences in species behaviour, this Knip et al.: Sharks in nearshore environments 3 paper will present a second population model and Rhizoprionodon terraenovae, Australian sharpnose some exceptions to Springer’s theoretical population. Rhizoprionodon taylori, bonnethead Sphyrna tiburo, For the purpose of this review paper, discussion will be and blacknose Carcharhinus acronotus sharks. These limited to sharks and will not include skate or ray spe- species can occur in nearshore waters for the duration cies; the term ‘nearshore’
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