Seal-fishery operational interactions: Identifying the environmentaland operational aspects of a trawl fishery that contribute to by-catchand mortality of Australian fur seals (Arctocephalus pusillusdoriferus)

摘要

Australian fur seals are known to interact directly (i.e. operational interactions) with trawlers fishing in the winter blue grenadier trawl fishery of western Tasmania, Australia. The purpose of this study was to identify the environmental and operational aspects of the fishery that were associated with increased numbers of seals observed at the surface. The incidence of net entry was determined in order to establish the effectiveness of the currently used Seal Exclusion Device (SED) at reducing by-catch and mortalities. In addition, the stage and depth at which seals were at greatest risk of entering the net at depth and becoming by-catch were also identified. An increase in seal numbers observed at the surface is assumed to be proportional to the increased risk of by-catch and mortality incidences at trawlers. The most important subset of environmental and operational factors for predicting the number of seals observed at the surface both in the fishing ground in general and during fishing operations were identified using Generalised Linear Modelling. The final model yielded a significant result (R super(2) = 0.63, P - 0.01, n = 149) and indicated that the number of seals observed increased when weather conditions deteriorated, particularly when barometric pressure decreased and when swell height and visibility increased. Vessel operations also influenced the number of seals observed; seal numbers increased when the number of nearby vessels and trawl frequency increased, but decreased when vessel speed increased. Seal numbers also increased as the distance from the nearest breeding colony and haul-out site decreased. Seal numbers at the surface generally increased steadily during trawling operations (n = 475), although brief declines were noted during shooting and hauling phases. Sub-surface net interactions were also examined using a submersible video camera unit. Seal activity at depth was converse to that observed at the surface, with increased seal numbers noted during shooting and hauling, suggesting that seals dived to forage on fish in the submerged net. All seal by-catch occurred during the day and almost half occurred during shooting. Mortalities were significantly higher during shooting compared with hauling (Fishers exact test: coef. = 0.65, P - 0.05). However, mortality rates were similar between tows with the Seal Exclusion Device (SED) attached and those without (Fishers exact test: coef. = 0.07, P - 0.99). Only one seal was detected entering and exiting the net mouth during monitored tows, suggesting that the recently observed reduction in by-catch levels (and mortalities) may not necessarily be attributed to the introduction of the Seal Exclusion Device. Seal by-catch recorded during hauling typically occurred when haul speeds exceeded minimum average swimming speeds for fur seals. Recommendations based on these findings have been made to assist with the future management of fur seal populations that interact with trawl fisheries.