MEPS prepress abstract  -  DOI: https://doi.org/10.3354/meps12442

Breeding failure of seabirds in relation to fish depletion: is there a single threshold of food abundance?

Magella Guillemette, François Grégoire, Dévrig Bouillet, Jean-François Rail, François Bolduc, Alain Caron, David Pelletier*

*Email: david.pelletier@cegep-rimouski.qc.ca

ABSTRACT: Various populations of seabirds are in decline across the world and one purported cause is the global collapse of fish stocks. Recently it has been shown that breeding success start to decline at some critical threshold (CT) of maximum prey abundance (MPA) for various species of seabirds across the world. Here we test this empirical threshold for Northern Gannets (Morus bassanus), a large seabird, and discuss its relevance under various conditions. Using a time series starting at the end of the 1970s, including estimated biomass of fish, gannet population and food consumed by gannets, we show a positive and non-linear relationship between fish biomass and breeding success. This correlation is 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 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 from fishing industry, the observed threshold is further reduced, indicating that such effects should be included in the context of ecosystem based management. Theoretical considerations are offered to explain further the discrepancy of our study with those of the meta-analysis, which includes the effect of diet on feeding profitability and the effect of population size.