UBC Theses and Dissertations
An ecological study and theoretical considerations of butter sole (Isopsetta isolepis) population in Hecate Strait. Kutty, Madasseri Krishnan
The ecology of the Hecate Strait population of butter sole (isopsetta isolepis (Lockington)) is studied to facilitate a better understanding of the fluctuations in abundance and to permit a more optimal utilisation of the stock. The reaction of the population to varying degrees of exploitation and natural mortality rates is analysed for steady and fluctuating recruitment using Ricker's model. The magnitude of error inherent in the models used for the theoretical studies of exploited populations is also examined by subjecting a hypothetical population in a steady state to various mortality and growth rates. This is done both for a continuous fishery and after appropriate modifications in Beverton's model for a seasonal fishery as well. Butter sole population spawning in Skidegate Inlet is confined to the Hecate Strait bank. The species show depth stratification, the young ones being found in shallower waters. Although the population exhibits seasonal movements between shallow and deeper waters, a north-south migration is limited mostly to the spawning population. Sexual differences in the time of onset of maturity and migratory pattern are also observed. No association between butter sole and related species of flatfish is noted. A study of the discreteness of the Hecate Strait population suggests that this area is inhabited by a single self-contained stock. Growth studies of butter sole indicate that there are annual, seasonal, regional and sexual differences. The average growth of butter sole belonging to a strong year class seems to be influenced by intra-specific competition, even though a tendency in later years to compensate for the initial difference in growth is also exhibited. The survival rate of butter sole above six years is relatively low. A comparison of the relative abundance of young butter sole taken in the 1952-1954 samples from along the Graham Island coast with the success and age composition of the fishery which existed from 1958-1960 when these year classes became fully exploitable, indicates that fluctuations in the butter sole population are mainly due to variations in early survival rate. Yield isopleths and equilibrium yield curves indicate that the maximum equilibrium yield will be obtained when the age of exploitation is 4.83 years or greater only when F is higher than 1.8. The fishing mortality operates for only a short time each year while natural mortality is continuous, and a change in the latter therefore influences the yield and biomass more than a similar change in the fishing mortality. A higher increase in the natural mortality with age results, under fluctuating recruitment, in greater deviations in yield and biomass. As the older age groups now contribute less to the population, the effects of fluctuations in abundance of the entering year class on the stock become more pronounced. There is little difference in the yield per recruit (Yw/R) estimates from Beverton's or Ricker's model for a hypothetical population under various situations, provided Ricker's model makes use of an exponential average of the biomass during each time period. The slight difference observed is due to the manner of depicting the growth pattern. Due to seasonal differences in growth rate, Yw/R estimate from Beverton's model, need not be more accurate than from Ricker's model. A heavier exploitation of the butter sole population is warranted on the basis of the theoretical studies, provided the heavier exploitation does not decrease the number of fertilized eggs by over-exploiting the males that are more abundant on the spawning grounds. Due to sexual differences in the population parameters and the migratory and recruitment patterns, the two sexes may be treated in further studies, as a special case of two competing populations exploited simultaneously.
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