The commercial exploitation of shrimp Palaemon serratus (Pennant) in Ireland

Abstract

Palaemon serratus is at the northern limit of its range in the British Isles. In Ireland it is most abundant in the southwest where it has been commercially fished since the mid-1970s. Landings in recent years have averaged between 200 and300 tonnes annually with an estimated export value of £2—3 m. These landings represent a three-fold expansion over those of the previous decade. The biology of the species was investigated over a 12 month period in Bantry Bay using commercial gear. At most times of the year there is a bimodal length frequency distribution and the life expectancy is interpreted as 2 years. Condition factor does not vary much during the year in males and immature females but the larger females put on up to 30% weight in the autumn. The reproductive cycle in Bantry resembles that in the south of England rather than that in north Wales, these two locations providing earlier studies of the species. The largest females come into berry in October and egg carriage within the population continues into the following summer; in May, a second group of smaller females, belonging to the 0 age group, carries eggs. Corroborative evidence for this interpretation is provided by the size of the ova and their developmental state. There would appear to be an influx of shrimp to Bantry Bay which builds up from May and declines after January but cohort and gender migrations are unclear. Catch per unit of fishing effort (cpue) is estimated from the weight of a consignment of shrimp delivered to a processor. Such data are variable but they are also consistent and stable over the short-term and throughout the range of shrimp fisheries. A time series from 1977 to 1994 suggests a 36% decline; the significance of this is not known. Shrimp fishing takes place during the autumn and winter months. In the southeast landings are taken throughout the year but those outside the period August to January, inclusive, do not exceed 8% of the total. In the southwest only 3% of landings are made outside these months, while in Connemara none was reported Mechanical grading in the factory is explored as a means of reconstructing age profiles. Two patterns of exploitation are described: that of the southwest and southern coast has a larger proportion of 0 group shrimp which may reach 40% by numbers of the total landings; in Connemara the proportion of 0 group shrimp is much smaller. Attempts are made to find some method of predicting aspects of the catch from biological and sea temperature data. There is a suggestion that a large brood year is influential in producing a successor whose size is estimated 2 years later. The sustainability of the shrimp fishery is unknown and two precautionary measures are suggested as the basis of a management regime: enlarging the mesh size to improve the exploitation pattern and limiting the fishing season.