Grey seal induced catch losses in the herring gillnet fisheries in the northern Baltic

• The interaction between grey seals (Halichoerus grypus) and the Baltic fishery for herring (Clupea harengus) was investigated for the period 2000-2004, using a three level perspective. Data from the official EU log-book system, data from a voluntary log book system, and data from field studies were combined. It was found, based on records from the official log-book and using a method with paired data compensating for temporal variations in catches and seal activity, that catches were significantly higher for fishing days when there were no seal interactions recorded during setting or lifting the nets, compared to days when there were such notations (0.59 and 1.03 kg /m net * day respectively; p< 0.001). It was found that the frequency of seal-disturbed fishing efforts, encompassing 30 % of all records of herring gillnet fishing in the official log-book, was probably an under-estimation and explained by the fact that making notes on seal interactions are optional for the fisherman. The corresponding figure of the occurrences of seal-disturbed fishing efforts was 60% in a voluntary log book system, which requires the contracted fishermen to record all occurring seal-interactions, in addition to detailed data on the whole of the fishing operation. There was a pronounced variation in the frequency of seal-disturbed fishing efforts in relation to the time of the year. The interaction was least in the early summer, and reached a maximum at the end of the year. The variation is alleged to be dependent on the life cycle of the seals and its prey, herring. It was found that the calculated seal-induced losses were larger than the occurring number of seals in the area reasonably could have consumed. It was therefore deducted that there was a significant hidden catch-reducing scaring effect from seals’ presence near the nets. The catches in the herring gillnet fishery decreased over the investigated period, whereas the catches in the trawling fishery increased, as revealed by the official log-book data. The variances in the catches were however too large to allow for an analysis of possible effects of seal interactions. The method that worked best for estimating the catch losses was using paired data which compensates for temporal variations in catches and intensity of seal interaction. A method using nets baited with marked fish for estimating the hidden losses was tested, but did not work well since seals removed more fish than the method could accept. Seals visited the experimental herring nets in 14 of the 19 trials. In 11 cases, more than 95% of the marked fish went missing. It is argued that the herring gillnet fishery in the north Baltic is severely affected by the seals-fisheries conflict.

Keyword

Conflict

catch loss

hidden damage

herring fishery

grey seal

Baltic Sea

NATURAL SCIENCES

NATURVETENSKAP

Publication and Content Type

ref (subject category)

art (subject category)