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Seabirds feed extensively on fisheries waste (discards and offal), and quantifying the degree of utilisation is an important and politically sensitive area of research (Furness 1992, Garthe & Hiippop 1994, Oro & Ruiz 1997). Studies on the feeding ecology of seabirds offer insights into issues like the energy flow in marine ecosystems and the ecological impacts of fishing activities. Recently, consulnption experiments have been combined with published data on fisheries discards and bird bioenergetics to estimate the number of seabirds sustained by feeding exclusively on fisheries waste (Garthe et al. 1996, Walter & Becker 1997). Here, I point out some of the estimation problems faced in such studies and recommend ways for improving the data analysis. In particular, I demonstrate how the Delta method (Seber 1982) can provide estimates of precision, thus allowing more informed decision-making by conservation and fisheries managers. Consumption of fisheries waste by seabirds is usually estimated by experiments conducted on board research vessels. In these experiments, items normally found in the discarded component of the colnmercial catch are discharged individually, and their fate is recorded. The realism of this design has been questioned for some time, mainly because experimental discarding cannot reproduce the conditions during commercial fishing (Garthe & Hiippop 1994). For example, Garthe & Huppop (1998) recently showed that discarding up to 50 items simultaneously resulted in significantly lower estimates of consumption rates than those obtained from single release experiments. Data from the Scottish discard sampling programme (Stratoudakis 1997, Stratoudakis et al. 1998) suggest that multiple releases are closer to commercial discarding practices, where large quantities of fish are processed per haul (median of 1200 fish discarded per haul in the Scottish demersal fisheries). Despite these deficiencies, the results of feeding experiments are used to quantify the total consumption of fisheries waste by seabirds in a fishery. Estimates of the number of seabirds (N) potentially sustained by feeding on fisheries waste are usually obtained by: (1) multiplying estimates of total discards (D) for a fishery by an average estimate of consumption rate (CR) to get the total biomass potentially available to seabirds; (2) multiplying this biomass by a calorific value (CAL) to obtain an energetic equivalent; (3) multiplying the energetic equivalent by the assimilation efficiency (AE) to get the potentially utilised part of this food source; and (4) dividing the potentially utilised energy by the average annual energetic requirement (ER) of a typical bird in the studied conlmunity:

Recommendations for improving the estimation of the number of seabirds potentially sustained by fisheries discards