Long‐term changes in the trophic level of the Celtic Sea fish community and fish market price distribution

Abstract

Summary1. The intensive exploitation of fish communities often leads to substantial reductions in the abundance of target species, with ramifications for the structure and stability of the ecosystem as a whole. 2. We explored changes in the mean trophic level of the Celtic Sea (ICES divisions VII f-j) fish community using commercial landings, survey data and estimates of trophic level derived from the analysis of nitrogen stable isotopes. 3. Our analyses showed that there has been a significant decline in the mean trophic level of survey catches from 1982 to 2000 and a decline in the trophic level of landings from 1946 to 1998. 4. The decline in mean trophic level through time resulted from a reduction in the abundance of large piscivorous fishes and an increase in smaller pelagic species which feed at a lower trophic level. 5. Similar patterns of decline in the trophic level of both catches and landings imply that there have been substantial changes in the underlying structure of the Celtic Sea fish community and not simply a change in fishery preferences. 6. We suggest that the reported changes in trophic structure result from reductions in the spawning stock biomass of traditional target species associated with intensive fishing, together with long-term climate variability. 7. The relative distribution of fish market prices has changed significantly over the past 22 years, with high trophic level species experiencing greater price rises than lower trophic level species. 8. Although decreased abundance of high trophic level species will ultimately have negative economic consequences, the reduction in mean trophic level of the fish community as a whole may allow the system to sustain higher fishery yields. 9. Management objectives in this fishery will depend on the relative values that society attaches to economic profit and protein production.

Feeding ecology of five commercial shark species of the Celtic Sea through stable isotope and trace metal analysis

“…Studies of stomach contents have been carried out for various species of sharks (Cortès, 1999;Ellis et al, 1996) but few references on their isotopic composition are yet available in the literature (Estrada, Rice, Lutcavage, & Skomal, 2003;Fisk, Tittlemier, Pranschke, & Norstrom, 2002;Pinnegar et al, 2002). In this study, we determined the d 13 C and d 15 N values of five shark species from the Celtic Sea, the tope shark Galeorhinus galeus, the black-mouthed catshark Galeus melastomus, the starry smooth hound Mustelus asterias, the spiny dogfish Squalus acanthias and the lesser-spotted dogfish Scyliorhinus canicula to trace their feeding habits, using also their stomach contents and the isotopic composition of their potential preys previously recorded in other studies (Ellis et al, 1996;Pinnegar et al, 2002).…”

“…In this study, we determined the d 13 C and d 15 N values of five shark species from the Celtic Sea, the tope shark Galeorhinus galeus, the black-mouthed catshark Galeus melastomus, the starry smooth hound Mustelus asterias, the spiny dogfish Squalus acanthias and the lesser-spotted dogfish Scyliorhinus canicula to trace their feeding habits, using also their stomach contents and the isotopic composition of their potential preys previously recorded in other studies (Ellis et al, 1996;Pinnegar et al, 2002). Zinc (Zn), lead (Pb), nickel (Ni), cadmium (Cd), iron (Fe), chromium (Cr), copper (Cu), selenium (Se) and mercury (Hg) concentrations were also examined for evidence of species differences, diet transfer, specific biomagnification processes.…”

“…The isotopic composition of an animalÕs tissue therefore reflects that of its prey with a slight trophic enrichment in 13 C and 15 N of about 1& and 3.4&, respectively (De Niro & Epstein, 1978Peterson & Fry, 1987). d 15 N ratios are often used as a predictor of relative trophic level (Pinnegar, Jennings, OÕBrien, & Polunin, 2002;Post, 2002;Vander Zanden, Cabana, & Rasmussen, 1997) …”

Defining value per unit effort in mixed métier fisheries

“…The achievable price of a species or group of species will determine the level of investment fishers are prepared to make to catch it (Pinnegar et al, 2002), or whether they attempt to catch it at all (Bastardie et al, 2013). Fishers may adopt alternative strategies that are perceived to be more profitable given species prices and predicted catch value (Marchal et al, 2007).…”

“…supply, demand and quality determine price at first sale (Abernethy et al, 2010;Bastardie et al, 2013;Pinnegar et al, 2002). Previous research into price variability suggests that in many fisheries prices are relatively inelastic to supply and vice versa given by weak correlations between catch volume and achieved price (Swartz et al, 2013).…”