Aspects of the biology of grey mullet, Mugil cephalus, in Lagos lagoon, Nigeria

Investigations were conducted in Lagos lagoon, Nigeria between February 2009 andJanuary 2010 on the biological aspects of grey mullet, Mugil cephalus with aim that informationobtained from the study will contribute to the baseline data for carrying future studies on its ecologyand conservation and development of its fisheries in this water body in particular and the Gulf ofGuinea in general. M. cephalus is a commercially valued fish in Nigeria and a member of the FamilyMugilidae. It is a catadromous fish species and widely reported in fresh, estuarine and brackishwaters. In this study, the occurrence of this fish was both seasonal and salinity dependence withhighest occurrence of 29.04% of the total catch when salinity was 20%o. 1+, 2+ and 3+ years’ old ageclasses were encountered. Fish ranged between 9.0 and 32.0 cmTL and weighed 12 and 345gBWrespectively. The length-weight relationships were estimated as LogW= -11.551+ 2.968 Log L(r=0.001) for males and LogW= -11.249 + 2.929 Log L(r=0.993) for females. The estimated growthparameters of the von Bertanlaffy equation L∞ =37.0 (±0.22) cm, K = 0.22.year-1, and to = -1.8year. The pattern of growth was near isometric with b=2.98 and 2.926 for males and femalesrespectively. The diet composition of this species included nine (9) groups of food items. Occurrencesof the diets varied monthly. The most foods were organic detritus and sand grains occurring in 40.60and 85.66% of the stomachs respectively. The presence of algae in the stomachs was an indication ofherbivorous habit of this fish, while occurrence of protozoans, crustaceans, nematodes, chaetognaths,and juveniles of other fish was indication of its carnivorous tendency, however, presence of detrituspresumed the fish as an omnivore. The overall sex ratio was 1 male to 1.09 female. A chi-squarerevealed a significant departure from the theoretical 1:1 sex ratio (X2 =17.662 > X21, 0.05=3.84).Fecundity varied from 620,320 to 1, 082, 200 eggs (801,300 ±2,000 eggs) for fish measuring 20.1cm TL(60.3gBW) and 32.0cmTL (345gBW) respectively. In this study the biological aspects of greymullet, M. cephalus in Lagos lagoon were discussed.