|Belgian shrimp research in 1973|
De Clerck, R.; Redant, F. (1975). Belgian shrimp research in 1973. Ann. Biol. (ISSN 0970-0153) 30: 214-215
In: Annals of biology. Bio-Publishers: Ludhiana. ISSN 0970-0153, meer
|Ook gepubliceerd als |
- De Clerck, R.; Redant, F. (1976). Belgian shrimp research in 1973, in: (1976). IZWO Coll. Rep. 6(1976). IZWO Collected Reprints, 6: pp. chapter 12, meer
In 1973 the commercial shrimp catches reached the very high level of 1615 tons. This represented the highest catch since 1956 and twice the landings of 1971 or 1972. The average engine-power of the vessels was 147.3 hp and the average catch per hour fishing was 23.6 kg. The catch curve shows the same pattern as the previous years, with a marked peak in October. This peak however was in 1973 (51 kg/ hour fishing) clearly much higher than in 1971 (33 kg/hour fishing) or in 1972 (30 kg/hour fishing). The experimental shrimp fishery was carried out by the RV Hinders. On the fishing ground of the Westdiep five stations were monthly sampled with an otter trawl. The shrimp catches were divided into two length classes (smaller or larger than 54 mm) and expressed in numbers per hour fishing. From these rough data monthly mean values for the five stations were calculated. The curve for shrimps smaller than 54 mm shows two distinct peaks: the first one in the period March/April 1973 (males and females born in the summer of 1972 and males born in the spring of 1972) and the second one in the period September/October 1973 (males and females born in the spring of 1973 and males born in the summer of 1972) (ICES CM 1972/K: 23). The curve for shrimps larger than 54 mm shows one significant peak in the period September/October 1973 and a hardly notable peak in the period March/April1973. In both cases, shrimps of 1 year and older are concerned. The very high densities observed for both length classes show that the shrimps population had recovered completely from the critical period, 1970 to 1971. The large density of the shrimp stock and the attendant high landings are due to different interdependent factors. The densities of cod along the Belgian coast during autumn and winter were considerably lower in 1972 than in 1970 or 1971. Thus the mortality of shrimps by predation decreased to a large degree. Consequently more shrimps grew up to sexual maturity and reproduced. This is reflected in the strength of the different succesive recruitments, which show a remarkable increasing trend (Annls biol. Copenh. 29: 186). The water temperature during summer was also important. High temperatures accelerate moulting frequency and thus also the growth. During the period May/August 1973 (resp. 1972) the average bottom temperature was 14.7 °C (resp. 13.7 °C). The higher temperature observed during the summer of 1973 probably explains why the peaks (both for the undersized and for the sized shrimps) were noted somewhat earlier this year than during the previous years (both about 1 month earlier).