Linking Large- and Small-Scale Fish Surveys with the Feeding Ecology of Seabirds

ICES CM 2007/R:04 Not to be cited without prior reference to the author Linking large- and small-scale fish surveys with the feeding ecology of seabirds Andreas Dänhardt & Peter H. Becker ABSTRACT The state of North Sea fish populations and stocks is annually assessed by means of extensive survey programmes as the basis for fisheries management. Yet since standard research vessels are restricted to deeper water, shallow coastal areas of the North Sea ecosystem pivotal to particular stages in the life cycles of target and non-target species are not covered. Local surveys are often limited in space and/ or time and are not coupled to the regular North Sea wide surveys. The Wadden Sea is a large intertidal zone, serving many ecologically and economically important fish species as a nursery, including herring, sprat, plaice, sole, cod and whiting, which together account for more than 75 % in weight of annual catches North Sea wide. In addition to their meaning to recruitment processes to the adult stock (and eventually to the fishery), the juvenile stages of these species also provide the nutritional basis for piscivorous seabirds that breed in large numbers along the North Sea coasts and have synchronized their reproductive phenology with the availability of their prey fish. Based on seasonal, diurnal and tidal cycles of fish occurrence we describe principles for monitoring fish in the Wadden Sea considering relevant phenological aspects. We suggest a link with offshore sampling programmes and propose an integration of the large- and small-scale survey programmes to be put into context with seabird feeding ecology. Contact author: Andreas Dänhardt, Institute of Avian Research, c/o Research Center Terramare, Schleusenstraße 1, 26382 Wilhelmshaven, Germany. Tel: +49 42 21 94 42 05, cell: +49 1 79 5 17 15 36, fax: +49 44 21 94 42 99, e-mail: [email protected] INTRODUCTION To define how and where human activities affect ecosystems is subject to a high level of uncertainty. Consequently, precautionary approaches in terms of human activities are required, and the levels of precaution should be proportional to the amount of information available such that the less is known about a system, the more precautionary management decisions should be (Marasco et al. 2007). These approaches may be perceived as overprotection unnecessarily restricting the utilization of natural goods and services while reducing the credibility of scientific advice (Langton et al. 1996), and are thus often not applied. Uncertainty can be reduced by identifying and quantifying functional relationships and food web interactions with special emphasis on spatio-temporal variation of biological (migrations, predator-prey overlap) and physical (variability of climatic- oceanic conditions at different scales) parameters. The North Sea is among the most heavily fished seas world-wide, and every year the state of fish populations and stocks is assessed by means of extensive survey programmes as the basis for managing fisheries (Flöter & Temming 2003). Yet components of the North Sea ecosystem pivotal to particular stages in the life cycles of target as well as non-target species -especially shallow coastal areas- are not covered, since large standard research vessels are restricted to deeper water. Local surveys are limited in space and/ or time (Vorberg et al. 2005) and are not coupled to the regular North Sea wide surveys. Moreover, they do not equally represent pelagic and benthic elements of the fish fauna. The Wadden Sea is Europe’s largest marine wetland and provides essential habitat for the early life stages of many ecologically and economically important fish species (Zijlstra 1978, Lozan et al. 2003), such as herring Clupea harengus, sprat Sprattus sprattus, plaice Pleuronectes platessa, sole Solea solea, cod Gadus morhua and whiting Merlangius merlangus, which together account for more than 75 % of annual catches North Sea wide (2003: 1 253 500 t, 176 500 t, 141 300 t, 17 900 t, 78 000 t and 43 200 t, respectively, Gröhsler and Zimmermann 2003). Due to its great productivity and area (22 000 km², Marencic and Essink 2005) the Wadden Sea not only provides an important nursery for the early life stages of many fish species, but also feeding and breeding habitat for a large number of migratory and breeding birds (Koffijberg et al. 2005, Blew et al. 2005), many of which utilise small fish as their main food source. Regular counts of breeding and migratory birds are carried out as part of the Trilateral Monitoring and Assessment Program (TMAG 1997), but this unique inventory does not account for the causes of the variability observed. In addition to time series of animals’ abundance, which are only able to show the effects of circumstances already affecting the monitored organisms, the detection and interpretation of (and possible reactions to) change requires the regular and long-term investigation of specific life history parameters and their responsiveness to changes e. g. food availability (Thyen et al. 1998, Becker 2003). For long-lived animals, such as most seabirds, life history theory predicts a reduction of their investment in current reproduction in years of poor food supplies, increased predation or bad weather in order to increase residual reproductive value and maximise lifetime reproductive success (Stearns 1992), so the surveillance of selected seabird colonies that are representative for populations may serve as an early warning system (Thyen et al. 1998, Furness et al. 2003). A number of traits in the life history of seabirds have been assigned suitability for monitoring environmental change (Tasker & Furness 2003), including breeding success (Croxall and Rothery 1991), variation in egg laying date (Becker 1996), egg size (Pons 1992), clutch size (Monaghan et al. 1992, Becker 1998), chick provisioning rates (Uttley 1992, Uttley et al. 1994), chick growth rates (Greenstreet et al. 1999), intensity of kleptoparasitism (Ludwigs 1998), adult and fledgling body mass (Becker et al. 2001a) and survival rates of adults and subadults (Furness et al. 2003). The extent to which food abundance is reflected by reproductive success is not uniform among seabird species, ranging from relative indifference (e. g. Northern Fulmar Fulmarus glacialis , Common Guillemot Uria aalge ) to high sensitivity (e. g. Common Tern Sterna hirundo ) to environmental variation. Based on seasonal, diurnal and tidal cycles of fish occurrence we describe principles for monitoring fish in the Wadden Sea considering relevant phenological aspects. We suggest a possible link with offshore sampling programmes and propose an integration of the large- and small-scale survey programmes to be put into context with seabird feeding ecology. MATERIAL & METHODS Data on fish species composition and length class distribution (not considered in this paper) were obtained from two sources: 1. A stow net was operated at least twice per month (depending on wind conditions) with an anchoring stow net cutter in the central Jade Bay (53°28 N, 08°12 E, 4 in Fig. 1) and south-east of the Wadden Sea island Minsener Oog (53°44 N, 08°02 E, 3 in Fig. 1) between April 20 th and September 5 th 2006. The mouth opening was 5 x 7 m, the mesh size (between knots) decreased from 20 mm close to the mouth to 5 mm in the codend. Due the small size of the meshes, the resistance of the filtering net surface against the water current caused the lower beam to be lifted, so that the vertical net opening changed as a function of the current velocity, varying between 4 and 5 m. Absolute catch numbers were thus normalized to individuals per 10 000 m 3 filterend water volume. Catches were obtained from the water surface down to 4-5 m at absolute water depths of 5-8 m, depending on the tidal hour, so that pelagic fish is thought to be represented well, demersal fish however may not have been quantitavely sampled. 2. Fish retained by trash screens at the cooling water intake of a power plant at the Jade, Northern Germany (53°33 N, 08°09 E, 5 in Fig. 1) were sampled between January 3 rd and December 19 th 2006. The cooling water was extracted directly from the Jade water body at a depth of 9 m below average low water level. The water volume taken by the power plants’ turbine was quite variable and due to comparability reasons with the stow net catches and literature data, fish collected from the cooling water were also expressed as n 10 000 m -3. As the basis for the seasonal variability of the Wadden Sea fish fauna, sampling at the cooling water intake was carried out fortnightly between January and March and September and December, and twice per week between April and August. On four occasions (August 1 st and 2 nd 2005, October 25 th and 26 th 2005, July 19 th and 20 th 2006 and October 24 th and 25 th 2006) 24 h-samplings were conducted (one haul every 2 hours = 12 hauls within 24 hours) to investigate tidal and diurnal variation in fish abundance. Common (Sterna hirundo ) and Arctic Terns ( Sterna paradisaea ) were used as representative examples of fish eating seabirds. The breeding biology in two of the largest tern colonies along the Lower Saxon Wadden Sea coast (Minsener Oog, 53°45 N, 08°.01 E, at the outer Jade and Banter See, 53°30 N, 08°05 E, in Wilhelmshaven, 1 and 2 in Fig. 1, respectively) was investigated. Parameters on breeding phenology such as first arrivals, arrival peak, first eggs, egg peak, first chicks hatched, hatching peak, first fledglings, fledging peak and breakup of the colonies were noted at the colony in Wilhelmshaven for Common Terns. At the colony site on Minsener Oog prey fed to Arctic Tern chicks was identified to lowest possible taxonomic level and the size of prey organisms relative to the average beak length (Glutz v.

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