Sexual maturity and spawning biomass of the cod stock on flemish cap (division 3M)

Abstract

In recent years, in Flemish Cap cod has been observed a progressive reduction on age and size at maturity from 1992 to 1995 (Saborido-Rey and Junquera, 1998). This reduction was probably a response to low population abundance and accordingly a certain recovering of the spawning biomass would be expected. However there was a lack of incoming recruitment in the latest years, which make the biomass of the Flemish Cap cod being in the lowest value recorded. Despite of the improvement in the scientific advice with the incorporation of new models in terms of uncertainty, the lack of biological realism can produce an inappropriate model assumptions, as for example those concerned with static life history parameters (Helser and Brodziak, 199-8). Several studies conftrm that population density is an important mechanism in regulating the dynamics of marine ftsh populations (TrippeL 1995; Tyler et aI., 1997) affecting to several life history parameters, as growth and age/size and maturity (Roff, 1984) and hence the possibility of the spawning biomass to increase in order to permit the rebuilding of the stock. Size and age at maturity of the Flemish Cap female cod has been determined by an histological method since 1990 at summer, when the ED-survey is carried out. The method uses cod ovaries in the cortical alveoli stage as an index of the onset of the oocyte development for the next breeding season, whereas the presence of postovulatory follicles in the ovaries shows the already spawned females (Zamarro et aI., 1993). The cortical alveoli are the ftrststructures to appear during the phase of oocyte growth (Wallace and Selman, 1981), and therefore they are the ftrst sign of the start of the female ripening before the beginning of the vitellogenesis. The postovulatory follicles are the structures left in the ovary after maturation is ftnished and oocyte ovulation. The spawning season of the. Flemish Cap cod is known to be short in time and the earliest in the year of all cod stocks in the Northwest Atlantic (Myers et aI., 1993); it occurs around March. This method allow to accurately detect the proportion of both recruit and repeat mature females at the very beginning of the gonad development, as it happens in summer, few months after spawning, when it is very difficult to make a visual diagnosis of the sexual maturity stage (Morrison, 1990). In this paper the proportion of females with postovulatory follicles in July 1998 is used to determine the size and age at maturity in 1996 and the proportion of females in the cortical alveoli stage to estimate the same parameters for 1999. Estimation of the spawning stock biomass is calculated and its evolution in the period 1990-1998 discussed