Potential variability in the paternal contribution to stock reproductive potential of northeast Arctic cod (Gadus morhua)

Abstract

There has been a growing interest in determining the stock reproductive potential (SRP) as a means to better understand the recruitment dynamics of a fish population. The focus in SRP studies has, with a few notable exceptions, been on numbers, maturation and fecundity of females and thereby total egg production (TEP). In the past the SRP of northeast Arctic cod (Gadus morhua) was estimated from TEP over the years 1946 to 2005. In this paper we present estimates of the corresponding total viable sperm production (VSP) based on sperm characteristics from published literature on other cod stocks. There have been large changes in VSP, which to a certain extent reflect changes in the mature biomass of the stock. There was a relatively large variability in the relationship between VSP and TEP in this stock. Since 1946 there have also been changes in mean length of mature males and females with a tendency toward smaller fish in the most recent time period. With the relatively large decline in mean size of mature females and the tendency to mature at a smaller size in the latter years the mature fish of both sexes are now of a similar mean size. The relationship between mean size of mature males and females is substantially different than when the stock was large in the early part of the time series. This could have implications for fertilization success, a factor that is dependent on the dynamics of both sexes in relation to each other. Utilisation of fertilization rates based on the sex ratio and sperm fertilization potential based on the condition of the fish allowed the TEP to be adjusted to an 'estimate' of total fertilized eggs, thus including both male and female characteristics in an estimate of SRP. The resulting fertilized egg to recruit plot showed a similar degree of variability as the SSB to recruit plot, however, the pattern was slightly different.