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

We compared the population dynamics of two geographically discrete stocks o f 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 Briti sh Columbia near the southern limit of the species' range. Rock sole in Hec ate Strait had a higher rate of growth and natural mortality, matured earli er and had a higher reproductive potential than rock sole in the Baring Sea . The difference in the temperature regime probably contributes to the slow er 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 decl ined at high stock sizes. We suggest that, in the case of rock sole in Heca te Strait density dependence could result from competition for food and hab itat at the larval and juvenile stages. The species exhibits highly localis ed 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. Thi s may have severe long-term consequences especially if environmental condit ions are unfavourable for recruitment. We examined the effect of temperatur e at the time of spawning on subsequent recruitment for both stocks, plots of temperature recorded at the time of egg and larval development vs, subse quent recruitment in the Bering Sea did not show a discernible relationship , although it was a significant component in a Ricker formulation of spawne r-recruit analysis. Temperature had a moderating influence on rock sole yea r-class strength in Hecate Strait but could not be considered a controlling factor. There was no synchrony in year-class strength between the two stoc ks suggesting that the two stocks an responding differently to environmenta l forcing factors and/or differences in spawning stock biomass. Target fish ing 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, high er yield was associated with a given rate of fishing mortality for the Heca te Strait stock compared to the Bering Sea stock. A fixed target fishing mo rtality reference point that is sustainable for the Hecate Strait stock cou ld result in an overfishing situation and a significant decline in abundanc e for the Bering Sea stock. The highest sustainable yield occurred at a fis hing rate slightly above F-0.1 for both stocks. (C) 2000 Elsevier Science B .V. All rights reserved.