Population dynamics of rock sole ( Lepidopsetta bilineata) in the North Pacific

Abstract

We compared the population dynamics of two geographically discrete stocks of rock sole ( Lepidopsetta bilineata) one in the Bering Sea off Alaska near the centre of the geographic range and the other in Hecate Strait off British Columbia near the southern limit of the species’ range. Rock sole in Hecate Strait had a higher rate of growth and natural mortality, matured earlier and had a higher reproductive potential than rock sole in the Bering Sea. The difference in the temperature regime probably contributes to the slower growth rate and longer time to maturity for the Bering Sea stock. There was evidence of density dependence in the relationship between spawners and recruits mitigated by ocean temperature for both stocks. Productivity declined at high stock sizes. We suggest that, in the case of rock sole in Hecate Strait density dependence could result from competition for food and habitat at the larval and juvenile stages. The species exhibits highly localised spawning which results in increased competition for a very limited food resource and prime habitat. In the case of Bering Sea rock sole production at high stock densities was impaired more than it was in Hecate Strait. This may have severe long-term consequences especially if environmental conditions are unfavourable for recruitment. We examined the effect of temperature at the time of spawning on subsequent recruitment for both stocks. Plots of temperature recorded at the time of egg and larval development vs. subsequent recruitment in the Bering Sea did not show a discernible relationship, although it was a significant component in a Ricker formulation of spawner-recruit analysis. Temperature had a moderating influence on rock sole year-class strength in Hecate Strait but could not be considered a controlling factor. There was no synchrony in year-class strength between the two stocks suggesting that the two stocks are responding differently to environmental forcing factors and/or differences in spawning stock biomass. Target fishing mortality reference points for F 0.1 and F med were higher for the rock sole stock in Hecate Strait than for the Bering Sea stock. Similarly, higher yield was associated with a given rate of fishing mortality for the Hecate Strait stock compared to the Bering Sea stock. A fixed target fishing mortality reference point that is sustainable for the Hecate Strait stock could result in an overfishing situation and a significant decline in abundance for the Bering Sea stock. The highest sustainable yield occurred at a fishing rate slightly above F 0.1 for both stocks.