Responses of seabirds to depletion of food fish stocks

Life history theory predicts that seabirds will respond to reduction in food abundance by changes in behaviour or breeding effort, buffering adult survival. Empirical data show some support for this but also sometimes indicate a trade-off in which survival of breeding seabirds may be reduced by food shortage. The sensitivity of seabird adult survival rates may be a feature of the detailed ecology of particular species and may be affected by ecological conditions such as the possibilities for prey switching. Fishery managers may set a lower limit biomass to protect fish stock recruitment, often at about 20% of predicted unfished stock biomass. It is unclear whether this threshold would also protect the needs of seabirds dependent on the fish stock. Time series of seabird breeding success and fish stock biomass may indicate minimum densities of food required. These are orders of magnitude more than the consumption by seabird populations. The critical prey density may also vary tremendously among seabird species and is clearly a function of the detailed ecology of each species. It is possible to predict which species will be most sensitive to reduced food supply. Sensitive species may be sentinels of the health'' of the marine ecosystem. For example, in Shetland and elsewhere in the North Sea, breeding success of kittiwakes Rissa tridactyla is particularly sensitive to abundance and quality of sandeels Ammodytes spp. However, seabird communities may be affected by a variety of interactions prompted by changes in fisheries; maintaining food fish levels may not alone be sufficient where communities have altered in composition over decades of fishing, as in the North Sea, and where predator-prey impacts induced by changes in fishery management may disrupt seabird communities.