Population ecology of dab (Limanda limanda L.) in the eastern Irish Sea, North Wales
Population density, growth and mortality rates, reproduction, feeding ecology and energy mobilization of dab (Limanda lmanda L) on the North Wales Coast have been investigated between 1998-2001. The total population number is estimated as 2.34 x 10 6. Average catch rates were lower in winterspring than in autumn-summer. 42 hr' in February 2000; 141 hr'in July 2000. Catch rates were generally higher at night than daylight: 1549 hr'at 02: 00 hr, 17 hr'at 10: 00 hr in November. The 2- year old fish form the dominant year class. There was seasonal variation in sex ratio (Females: Males). In the long-term data, in autumn, the females were more abundant (1.83: 1) in trawl catches but the contribution of the males increases in spring (1.15: 1). There were seasonal variations in length and weight relationships: the average relationship was nearly isometric (power bx3.0); in male W=0.01 x TL 91 and in female, W=0.01 x TL 3'04. April 1s' was established as the birthday of the fish and the greatest deposition of the opaque zone in the otolith occurred in summer. The male grows at a faster rate than female in autumn 2000: (Loo=25.7 cm; K= 0.30 y 1) and (loo=33.4 cm; K= 0.25 y-) respectively. The long-term average total mortality rate was significantly higher in males (Z= 1.95 y) than females (Z= 1.16 Y"). This difference was calculated to be largely due to the relatively higher fishing mortality rate on the males. There were cyclical cycles in the gonad, liver weight and condition factors. The peak spawning activity in males was in December-January whilst it occurred in February-March in females. In the 16-20 cm male, the maximum gonadocarcass ratio, GCR, (1.8 %) and hepatocarcass ratio, HCR, (2.9 %) were in January and August respectively. In female, the maximum GCR (17.1 %) and HCR (3.8 %) occurred in February. The GCR was at a minimum in spring-summer. The minimum HCR occurred in spring. The maximum condition factor (K) was in summer for male (K=0.97) and female (K=1.02) but, after spring spawning, declined to 0.67 and 0.72 respectively. The maximum average oocyte diameter (0.56-0.67 mm) was observed in fully ripe fish in the spawning season but, after spring spawning, reduced to between 0.10-0.22 mm. The total estimated egg production was 2.47 x 10 ' eggs but the average per mature female was 3.51 x 105 eggs. Relative fecundity declined from 3045 eggs g' in fish aged I to 1917 eggs g' in fish aged 8. The fecundity-total length (F= 41.6 TL IN) and fecunditysomatic weight (F= 4804SW 0.88) relationships are estimated to be good predictors of dab fecundity. In autumn 2000, the length (Lw%) and age (A w%) at 50 % maturity were lower in males (L = 13.3 cm; Ate, =1.3 yr. ) than females (L = 18.0 cm; A =2.4 yr. ). The long-term averages showed relative stability in the females but a decrease in males, Ate, from about 2 years to about 1.5 years. Five major taxonomic groups dominated the diet of dab: the ophiuroids, bivalves, crustaceans, polychaetes and hydroids. The ophiuroid, Amphiura brachiata, predominated. However, the study suggests size-selective preference for some prey groups. The composition of polychaetes in the diet declined with increasing size of the fish; 22.9 % by weight in the 11-15 cm length group to 5.7 % in the 26-30 cm group. On the contrary, the bivalves increase in the diet with increasing size of the fish, from 11.2 % (by weight) in the 11-15 cm group to 31.4 % in the 231cm group. There is however a significant diet overlap (C; >- 0.60) between the size groups. In general, daily food intake was higher in late spring-summer than in autumn-winter. In May, for example, daily food intake for the 21-25 cm and 26-30 cm female dab was estimated as 1.49 g and 4.61 g respectively. In February it was 0.52 g and 1.76 g respectively. There were distinct seasonal changes in lipid and energy composition in the liver. Lipid content was significantly higher in the summer-autumn period than spring season. In September, it was about 62 and 69 % in male and female livers respectively. After spring spawning, the composition declined to 28 and 24 % respectively. The energy content per dry weight of liver peaked in October (8.50 kcal g' and 8.61 kcal g') in male and female dab respectively. Also, after spring spawning, the energy levels declined to 6.18 and 5.86 kcal g'' respectively. The energy requirement for routine metabolism and gonad development in a 20 cm female is estimated as 111.3 kcal but energy gained from food intake and carcass and liver reserves is estimated as 117.9 kcal. In male, the total energy requirement was 83.4 kcal but energy gained from food intake and carcass and liver reserves is estimated as 113.3 kcal. Feeding (in late autumn and winter) and carcass tissues are the main suppliers of energy for activity and the spawning process.