Biology and population dynamics of the King Mackerel (Scombereomorous commerson, Lacepede, 1800) off the coast of Natal .

Abstract

This thesis provides a study on aspects of the biology and population dynamics of Scomberomorus commerson off the south east coast of southern Africa. This is necessary for the evaluation of the current management policies and for the selection of the "best" management strategy for this species off the coast of Natal. The current status of S. commerson off the Natal coast was assessed in terms of yield-per-recruit and spawning biomass-per-recruit analyses. Input parameters to these per-recruit models include growth and mortality rates and basic biological data. Estimates of these parameters and the methods employed are detailed in the thesis . S. commerson has a protracted spawn ing period rang ing from November to March. The principal spawning area is Mozambique. Fifty percent sexual maturity is attained at 1096mm and 706mm(FL) for females and males, respectively. The, ratio of males to females in the sampled catches is approximately 1:2 with females attaining a significantly larger mean size: females (926mm,FL) and males (898mm , FL). The masslength relationsh ip for both sexes can be described by the following equation: Ma s s ( g ) = O.1353X10-5 • [FL(mm) J3.25 15 Growth parameter estimates for the combined sexes were estimated from a lengthbased as well as an age-based method. An objective technique was utilised to determine which growth function best describes the age-length data of S. commerson. Age data were obtained from otolith readings. The precision of otolith readings was described by an index. This index, the average percent error, which is equal to 20.25%, is higher than that recorded in other studies. Two opaque bands are laid down annually. This was validated by marginal increment analyses as well as from tagging data. The age-length relationship , assuming biannual periodicity of the opaque band, is best described by a Von Bertalanffy growth function: L age(mm, FL) = 134 4mm (l-e ...{).292 yr-'Cage-+2.999 yrs») The instantaneous natural mortality rate (M) was estimated using two different techniques: the Pauly equation and the Rihkter and Efanov equation. The former equation was very sensitive to changes in the mean environmental temperature and both techniques produced different estimates. The average of both methods was, therefore, taken as an estimate of M which is 0.5 yr'. The instantaneous fishing mortality rate (F) is currently estimated to be 0.25 vt", This estimate is, however, positively biased as the effects of emigration have not been taken into account. The per-recru it analyses were conducted for three different growth equations for the same values of F and M, age-at-maturity and age-at-first-capture. For all growth equations the yield-per-recruit increased with increased fishing with maximum yield-perrecruit attained either at infinite F or at very high values of F (> 5 yr'), FO.1 was attained between 0.6 and 0.8 v' for all growth equations. The Von Bertalanffy growth function, assuming annual periodicity of the opaque band, was the most unrealistic. It predicted a virtual collapse of the fishery When F=M and a reduction of the spawning biomass to 50% of its unfished level at F=0.1 yr'. The length based derived growth equation and the Von Bertalanffy growth function, assuming biannual periodicity of the opaque band, predicted that spawning biomass dropped to 50% of the pristine level at F=0.19 and 0.16 vr' . respectively. It is believed that the current restrictions on sport catches of 10 fish/person/day offers adequate protection for the Natal king mackerel stock. These restrictions should be maintained in Natal. However, if fishing effort on this species continues to increase (as is anticipated in an open access fishery) or if there is increased commercial interest or if there is renewed fishing in Mozambique, a minimum size is recommended to adequately protect the spawning stock. Such a minimum size should be implemented in Mozambique which is the principal spawning area for king mackerel. Setting a minimum size in Natal, above the size-at-50% maturity may substantially reduce catches in the short and medium term because at least 90% of the catch currently taken will be inaccessible to fishermen. Restrictions of king mackerel catches in Natal, alone, is not considered a viable management option as Natal's commercial and to a certain extent recreational fishermen may turn to Mozambican waters to harvest king mackerel. Management options should be implemented and enforced both in Natal and Mozambique.