One of the main issues in fisheries management is assessing and reducing fishery effects on other predators such as marine mammals and seabirds (Ormerod 2003). Such effects may be direct, as in by-catch mortality (Tuck et al. 2001), or indirect through reductions in food supplies. During the 20th century, many seabird species showed large increases in population size in the north-east Atlantic (Mitchell et al. 2004). For some, these increases were probably linked to growing and changing human fisheries (Montevecchi 2002), either through provisioning of extra food in the form of discards and offal (Garthe, Camphuysen & Furness 1996) or through high fishing pressure on large predatory fish leading to increased availability of the smaller fish that usually form the main prey of seabirds (Sherman et al. 1981; Furness 1982). However, towards the end of the last century, the development of industrial fisheries harvesting small forage fish has led to direct competition between humans and seabirds. Regional declines have become apparent in some seabird species (Anker-Nilssen, Barrett & Krasnov 1997; Heubeck et al. 1999) and some have been related to human fisheries (Anker-Nilssen, Barrett & Krasnov 1997). Overexploitation has been suggested as the cause of population crashes of some forage fish species and concomitant declines in seabird populations during the last 50 years (Newton 1998).
The largest single-species industrial fishery in the North Sea harvests the lesser sandeel Ammodytes marinus Raitt, up to a million tonnes of which are landed each year (ICES 2003). Lesser sandeels are an important prey for most species of seabirds around the North Sea during the breeding season (Monaghan 1992; Wanless, Harris & Greenstreet 1998). Among these is the black-legged kittiwake Rissa tridactyla (L.), which in this region feeds almost exclusively on lesser sandeels (Harris & Wanless 1997; Lewis et al. 2001). Adult kittiwakes eat mostly older (1+ year group) sandeels during April and May, but switch to juvenile (0 year group) sandeels for both themselves and their young in June and July (Harris & Wanless 1997; Lewis et al. 2001). The number of kittiwakes breeding along the British North Sea coast declined by 20–50% between c. 1986 and 2000, with even stronger declines in Shetland (Heubeck et al. 1999; Heubeck 2004). The cause and mechanism of these declines are unclear, although consistently low breeding success was observed in Shetland and south-east Scotland (Mavor et al. 2003). The underlying cause is likely to be low availability of sandeels, but in the absence of long-term sandeel monitoring outside Shetland correlations have to be sought between kittiwake performance and proxies thought to be affecting sandeel abundance.
In 1990, a summer industrial fishery for sandeels started around the Wee Bankie off the Firth of Forth, south-east Scotland. Landings from this fishery quickly grew to 100 000 tonnes in 1993 but then stabilized at a lower level (see Appendix 1). Subsequent research indicated that the Wee Bankie fishery almost certainly caused local depletion of sandeel numbers, which coincided with reduced breeding success of kittiwakes (Rindorf, Wanless & Harris 2000). In 2000, following advice from the International Council for the Exploration of the Sea (ICES) Advisory Committee on the Marine Environment, summer fishing for sandeels was banned in 20 000 km2 of sea to the east of Scotland and north-east England, an area that included the Wee Bankie (Wright et al. 2002). A limited scientific fishery was maintained in the exclusion zone in 2000–01 (see Appendix 1).
Changes in oceanographic conditions can also exert a profound influence on ecosystem structure and hence top predator populations (Hunt et al. 2002). Physical and biological oceanographic conditions in the North Sea have changed markedly in recent decades (Beaugrand 2004), and high winter sea temperatures have been shown to reduce recruitment of sandeels, which spawn in winter (Arnott & Ruxton 2002). There is some evidence that seabird population dynamics in the region may be linked to ocean climate fluctuations (Thompson & Ollason 2001; Durant, Anker-Nilssen & Stenseth 2003). Previous studies of kittiwakes have shown a link between breeding success and prevailing weather patterns (Aebischer, Coulson & Colebrook 1990), but no study so far has investigated the combined effects of oceanography and fisheries on a North Sea seabird.
Since 1986, the Isle of May in the outer Firth of Forth has been part of the UK Joint Nature Conservation Committee (JNCC) seabird monitoring programme. Numbers, breeding success and adult survival of kittiwakes and other seabird species are monitored annually. Kittiwakes breeding on the Isle of May forage largely within the area targeted by the Wee Bankie sandeel fishery (Wanless, Harris & Greenstreet 1998; Daunt et al. 2002) and are thus potentially vulnerable to declines in sandeel abundance caused by the fishery. We used data collected between 1986 and 2002 to (i) estimate population parameters and model the growth of the Isle of May kittiwake population over this period; (ii) investigate to what extent the Wee Bankie sandeel fishery and oceanographic change are likely to have affected population growth; and (iii) predict future population growth under various scenarios of fishery and oceanographic conditions.