Aspects of the ecology of Gadoid Fish of the West Coast of Scotland

Abstract

The distribution, abundance, growth, length-weight relationships and reproductive ecology of whiting, Norway pout and poor-cod populations were studied in inshore and offshore areas around Oban from November 1974 to March 1977. Planktonic stages and mature adults were found only at offshore sites, the inshore areas being nursery grounds for all three species. Immigration occurred between June and December of the O-group when the fish were nektonic, with part of the population pelagiC and part demersal. In whiting the young fish aggregated for a short period inshore, but in other species abundance was always greater offshore. Norway pout and poor-cod remained on the nursery. grounds for between one and two years, whereas few whiting remained longer than one year. In all species shallower sampling sites were vacated more rapidly than deeper ones. The migration of fish offshore is considered in relation to growth, diet and maturation. The ecological significance and management implications of the migrations are discussed. The rate of growth in length and weight varied seasonally, being more rapid in summer and autumn in all species. Differences were found between localities in rate of growth and are thought to be due to a preference by older fish for deeper water. Only the poor-cod spawned in the study area, at Tiree Passage and on the south-west coast of Null. Spawning occurred between January and May, with a peak in April. Females matured at a mean.length of 15.6 cm in the 2-group with only a small proportion of 1-group fish maturing. In males the average age of first maturity was lower (14.0 - 14.6 cm) and a greater proportion of 1-group fish matured, which compensated for a slightly lower survival rate in older fish and equalised the sex-ratio of the spawning popUlation. The success of reproduction was highly dependent on a single year-class, since 83.3% of reproducing females belonged to the 2-group. Although ripe and spent whiting and Norway pout were caught at offshore sites no spawning fish were encountered. The possibility of a spawning migration out of the area and the locality of spawning is discussed for each species. Length-weight relationships were calculated for each species and significant differences were found between males and females in all cases. Seasonal variations in condition were studied by calculation of condition factors and by analysis of monthly length-weight relationships.