Modelling the impact of fishery by‐catches on albatross populations

Abstract

Summary1. Several albatross species, including the wandering albatross Diomedea exulans , have shown marked declines in abundance throughout their range. These seabirds are frequently taken as by-catch in longline fisheries and this mortality has been implicated in the population declines. 2. We developed a deterministic, density-dependent, age-structured model for assessing the effects of longlining on wandering albatross populations. We used demographic data from field studies at South Georgia and the Crozet Islands, data on albatross abundance from 1960 to 1995, and reported effort data from the tuna longline fisheries south of 30 ° S, to model estimated by-catch levels and other population parameters in the model. 3. The model used two alternative assumptions about patterns of at-sea distribution of wandering albatross (uniform between 30 ° S-60 ° S; proportional to the distribution of longline fishing effort between these latitudes). 4. Our model was able to predict reasonably closely the observed data from the Crozet Islands wandering albatross population, but the fit to the South Georgia population was substantially poorer. This probably reflects: (i) greater overlap in the Indian Ocean than in the Atlantic Ocean between the main areas of tuna longline fishing and the foraging ranges of wandering albatrosses from the Crozet Islands and South Georgia, respectively; and (ii) greater impact of poorly documented longline fisheries, especially the tuna fisheries in the south Atlantic and the Patagonian toothfish Dissostichus eleginoides fishery, within the foraging range of wandering albatrosses from South Georgia. 5. The model results suggest that the marked decline in both populations, and subsequent recovery of the Crozet Islands population (but not the continued decline of the South Georgia population), can be explained by the tuna longline by-catch. They further indicate that populations may be able to sustain some level of incidental take. However, the likely under-reporting of fishing effort (especially in non-tuna longline fisheries) and the delicate balance between a sustainable and unsustainable level of by-catch for these long-lived populations suggest great caution in any application of such findings.

Effects of climate change and fisheries bycatch on Southern Ocean seabirds: a review

“…Better documented are the direct interactions between seabirds and fisheries, and more particularly bycatch, which may have been implicated in population declines of several species of seabirds in the Southern Ocean (e.g. Weimerskirch et al 1997, Tuck et al 2001. However, the effect of bycatch on demographic parameters and population dynamics remains poorly known for several populations, and even less is known about the potential interactions between bycatch and climate on seabird population dynamics.…”

Stage-dependent distribution of the Critically Endangered Amsterdam albatross in relation to Economic Exclusive Zones

“…At-sea mortality induced by industrial fishing has been identified as the main factor currently affecting seabird populations worldwide (Tuck et al 2001, Lewison et al 2012. In this context, conservation efforts in fishing practice and regulatory meas-ures have been undertaken and/or proposed to mitigate incidental seabird capture at sea (e.g.…”

Global seabird bycatch in longline fisheries

“…However, quantifying the scale of the problem is difficult due to the diverse and remote nature of many of the world's fisheries, the lack of systematic reporting, and the nature of seabird bycatch rates themselves, which can be highly variable. Nevertheless, several reviews have concluded that recent and/or current reported levels of seabird bycatch are demographically unsustainable for the populations involved (Croxall et al 1998, Tuck et al 2001, Arnold et al 2006, Barbraud et al 2009, Rivalan et al 2010.…”