Breeding failure of seabirds in relation to fish depletion: Is there one universal threshold of food abundance?

Many populations of seabirds are in decline across the world and one proposed cause is the global collapse of fish stocks. Recently, it has been shown that breeding success starts to decline at some critical threshold (CT) of maximum prey abundance (MPA). Here, we estimated this empirical threshold for a large seabird, the northern gannet Morus bassanus, and discuss its relevance under various conditions. Using a time series starting at the end of the 1970s that included estimated biomass of fish, gannet population and food consumed by gannets, we found a positive and non-linear relationship between fish biomass and breeding success. This correlation was supported by the results of a bio-energetic model indicating that gannets extract a substantial fraction of the available biomass. In addition, distribution and time spent at sea (derived from GPS recordings) increased considerably when food abundance was low. In contrast to the general CT previously proposed (34%), we found that breeding success of gannets started to decline at about 8% of MPA. Moreover, when corrected for the sizes of prey available that may be eaten by the birds and the removal of fish by the fishing industry, the observed threshold was further reduced, indicating that such effects should be included in the context of ecosystem based management. Theoretical considerations are offered to further explain the discrepancy between the results of our study and a previously published meta-analysis, which included the effect of diet on feeding profitability and the effect of population size.