Cape fur seals trapped in trawling gear: Age structure, sex ratio, seasonality and distribution

Biological and distributional information on Cape fur seals Arctocephalus pusillus pusillus trapped in research and commercial trawl nets were examined and compared with data from seals shot during research cruises. The male/female ratio of 150:113 for all untagged seals trapped by research trawls did not differ significantly from the expected sex ratio of 53:47 for untagged seals on the colonies. However, more male than female tagged seals were trapped in commercial trawls (81 and 19 respectively), the sex ratio being significantly different from the expected ratio of 65:35 for harvested tagged seals. Most of the seals trapped in research trawls were caught during midwater trawls, but most seals trapped in commercial trawls were caught by the offshore trawling sector. Numbers of seals trapped increased with age, suggesting that scavenging from trawlers may be a learned behaviour. However, the number of mates trapped decreased after age 4 1/2 years, consistent with the natural decline in their numbers with age. The fewer seals trapped in summer (November - February) may be linked to their breeding cycle.