Mortality and biological reference points for the king mackerel (Scomberomorus commerson) fishery off Natal, South Africa (based on a per-recruit assessment)

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Abstract

Mortality estimates for the king mackerel (Scomberomorus commerson) and biological reference points derived from a yield and a spawning biomass-per-recruit assessment for the fishery off Natal, South Africa are presented. Total mortality was estimated from a catch curve and a technique requiring age-at-full requirement and mean age of fully recruited fish. The estimates of total mortality (Z) differed between these techniques and an average Z of 0.75 year-1 was assumed. The natural mortality rate (M) was also estimated from two different techniques and yielded different values. An average M = 0.5 year-1 was therefore assumed. The per-recruit model was based on three different growth curves and was sensitive to the choice of growth parameters. A growth curve assuming annual periodicity of the otolith bands predicts a virtual collapse of the fishery when the fishing mortality rate (F) equals M. At the current age-at-first-capture it also predicts that the spawning biomass will be 50% of its unexploited level at a fishing mortality rate of 0.1 year-1. This scenario is considered unrealistic for the Natal king mackerel fishery. The per-recruit analyses based on the other two growth curves show no indication of biological overexploitation at the current fishing mortality rate (0.25 year-1). Values of M from 0.3 to 0.4 and from 0.5 to 0.7 year-1 show little variation in the biological reference point estimates. The current spawning biomass is probably at a minimum value of 35% of its unfished level. Setting a minimum size above the length-at-maturity will result in short term deceases in yield but with substantial gains in spawning biomass-per-recruit. However, such a management strategy will result in socio-economic problems because at least 90% of the catch currently being taken will no longer be accessible.

Original languageEnglish
Pages (from-to)195-208
Number of pages14
JournalFisheries Research
Volume23
Issue number3-4
DOIs
Publication statusPublished - 1995

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Scomberomorus
mackerel
South Africa
fishery
fisheries
mortality
fishing mortality
growth curve
spawning
biomass
otolith
Africa
otoliths
periodicity
socioeconomics
methodology
fish

Keywords

  • Biological reference point
  • Mortality
  • Scomberomorus commerson
  • Yield-per-recruit

ASJC Scopus subject areas

  • Aquatic Science

Cite this

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title = "Mortality and biological reference points for the king mackerel (Scomberomorus commerson) fishery off Natal, South Africa (based on a per-recruit assessment)",
abstract = "Mortality estimates for the king mackerel (Scomberomorus commerson) and biological reference points derived from a yield and a spawning biomass-per-recruit assessment for the fishery off Natal, South Africa are presented. Total mortality was estimated from a catch curve and a technique requiring age-at-full requirement and mean age of fully recruited fish. The estimates of total mortality (Z) differed between these techniques and an average Z of 0.75 year-1 was assumed. The natural mortality rate (M) was also estimated from two different techniques and yielded different values. An average M = 0.5 year-1 was therefore assumed. The per-recruit model was based on three different growth curves and was sensitive to the choice of growth parameters. A growth curve assuming annual periodicity of the otolith bands predicts a virtual collapse of the fishery when the fishing mortality rate (F) equals M. At the current age-at-first-capture it also predicts that the spawning biomass will be 50{\%} of its unexploited level at a fishing mortality rate of 0.1 year-1. This scenario is considered unrealistic for the Natal king mackerel fishery. The per-recruit analyses based on the other two growth curves show no indication of biological overexploitation at the current fishing mortality rate (0.25 year-1). Values of M from 0.3 to 0.4 and from 0.5 to 0.7 year-1 show little variation in the biological reference point estimates. The current spawning biomass is probably at a minimum value of 35{\%} of its unfished level. Setting a minimum size above the length-at-maturity will result in short term deceases in yield but with substantial gains in spawning biomass-per-recruit. However, such a management strategy will result in socio-economic problems because at least 90{\%} of the catch currently being taken will no longer be accessible.",
keywords = "Biological reference point, Mortality, Scomberomorus commerson, Yield-per-recruit",
author = "Anesh Govender",
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N2 - Mortality estimates for the king mackerel (Scomberomorus commerson) and biological reference points derived from a yield and a spawning biomass-per-recruit assessment for the fishery off Natal, South Africa are presented. Total mortality was estimated from a catch curve and a technique requiring age-at-full requirement and mean age of fully recruited fish. The estimates of total mortality (Z) differed between these techniques and an average Z of 0.75 year-1 was assumed. The natural mortality rate (M) was also estimated from two different techniques and yielded different values. An average M = 0.5 year-1 was therefore assumed. The per-recruit model was based on three different growth curves and was sensitive to the choice of growth parameters. A growth curve assuming annual periodicity of the otolith bands predicts a virtual collapse of the fishery when the fishing mortality rate (F) equals M. At the current age-at-first-capture it also predicts that the spawning biomass will be 50% of its unexploited level at a fishing mortality rate of 0.1 year-1. This scenario is considered unrealistic for the Natal king mackerel fishery. The per-recruit analyses based on the other two growth curves show no indication of biological overexploitation at the current fishing mortality rate (0.25 year-1). Values of M from 0.3 to 0.4 and from 0.5 to 0.7 year-1 show little variation in the biological reference point estimates. The current spawning biomass is probably at a minimum value of 35% of its unfished level. Setting a minimum size above the length-at-maturity will result in short term deceases in yield but with substantial gains in spawning biomass-per-recruit. However, such a management strategy will result in socio-economic problems because at least 90% of the catch currently being taken will no longer be accessible.

AB - Mortality estimates for the king mackerel (Scomberomorus commerson) and biological reference points derived from a yield and a spawning biomass-per-recruit assessment for the fishery off Natal, South Africa are presented. Total mortality was estimated from a catch curve and a technique requiring age-at-full requirement and mean age of fully recruited fish. The estimates of total mortality (Z) differed between these techniques and an average Z of 0.75 year-1 was assumed. The natural mortality rate (M) was also estimated from two different techniques and yielded different values. An average M = 0.5 year-1 was therefore assumed. The per-recruit model was based on three different growth curves and was sensitive to the choice of growth parameters. A growth curve assuming annual periodicity of the otolith bands predicts a virtual collapse of the fishery when the fishing mortality rate (F) equals M. At the current age-at-first-capture it also predicts that the spawning biomass will be 50% of its unexploited level at a fishing mortality rate of 0.1 year-1. This scenario is considered unrealistic for the Natal king mackerel fishery. The per-recruit analyses based on the other two growth curves show no indication of biological overexploitation at the current fishing mortality rate (0.25 year-1). Values of M from 0.3 to 0.4 and from 0.5 to 0.7 year-1 show little variation in the biological reference point estimates. The current spawning biomass is probably at a minimum value of 35% of its unfished level. Setting a minimum size above the length-at-maturity will result in short term deceases in yield but with substantial gains in spawning biomass-per-recruit. However, such a management strategy will result in socio-economic problems because at least 90% of the catch currently being taken will no longer be accessible.

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