From mantle length frequency distributions of from four cruises on board of commercial fishing boats, carried out in the summers 1967, 1972 and 1978 and the winter 1976, we have arrived to the following conclusions:
1) Length distributions were split into age components, from which growth and mortality rates were analysed.1a) Growth of both sexes together can be described by the von Bertalanffy equation
lt = 32.15(1 - e-0,38(t + 0,34))
which is correct also for males (explanation in the text).Female growth can be described by the equation
lt = 24.5(1 - e-0,51(t + 0,32))
1b) Catch curves show a total mortality rate of 0.75, 1.29, 1.1 and 1.09 for the years 1967, 1972, 1976 and 1978 respectively, on six-monthly basis.1c) Knowing from CECAF (1979) the fishing effort of those years, we could delimit M values between 0.5 and 1.0 (yearly basis), although we believe the real value is nearer the lower limit specified than the higher one.
2) We have described the sex ratio by length curve, that allowed us to define a “knife edge mortality for females” at 22.25 cm on mantle length(2.75 years following the general equation). That differential mortality appears in the catch curves per sex.
3) Yield per recruit Y/R curves for 60 mm mesh size show a peak about F = 1 for M - 0.5. They are very levelled off, with Y/R very alike between F = 0.75 and F = 1.5, this is, more or less, mortality exerted in 1976; relative fecundity per recruit decreases between these two values by 47 percent. The predicted peak in yield per recruit corresponds roughly to the effort over the years 1967-1968.
For the 40 mm mesh size, used until 1976 by most of the boats in the fishery, Fmax is only a little lower than for the 60 mm mesh size, and the yield per recruit is still more level. However, the relative fecundity per recruit decreases dramatically between the same F values - by 67 percent.
4) A change in mesh size from 40 to 60 mm should cause - according to 1976 effort - a 30-40 percent increase in yield per recruit after about two years. Whatever may be the exploitation intensity, this change improves the yield per recruit.
5) Global production models - before, “production models” - show RMS between 100 000 t and 110 000 t, corresponding to an effort about 1 200 units as defined by the CECAF Working Group (1979); this effort is about 50 percent higher than for 1967, and much nearer that measured in 1972. This disagreement between yield per recruit and global models can be due to the integration in these latter models of the stock/ recruitment relationship, and the gradual expansion of the fishing zone during the historical period, which results in an optimistic version of the production models.
6) A lack of correlation in 1976 and 1977 between Japanese and Spanish cpue series can be shown; this fact could be caused by the change in fishing zone of the Japanese fleet, that should imply a new point on stock structure, and the change in the measurement effort by Spanish teams - effort now based on freezer trawlers -.
7) We should recommend not to increase fishing effort in this zone, at least before we know with more precision the stock/recruitment relationship and can apply techniques for the analysis of current populations.
