The biology of and fishery for king mackerel, Scomberomorus commerson (Scombridae), along the southern Mozambique and KwaZulu-Natal coast.
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Date
2013
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Abstract
The king mackerel, Scomberomorus commerson, is an epipelagic, schooling
predator supporting significant commercial, artisanal and recreational fisheries
throughout the coastal waters of its Indo-Pacific distribution. Despite the importance
of the species within the South West Indian Ocean, little research has been
undertaken on its biology and fisheries on a regional basis over the past 20 years.
The aim of this study was to evaluate the fishery and biology of S. commerson in the
South West Indian Ocean and identify gaps in information required to ensure its
effective management. Catch and effort data for the KwaZulu-Natal recreational S.
commerson linefishery were extracted from the National Marine Linefish System in
order to assess spatial and temporal trends in abundance. Generalized linear models
utilizing the delta method were used to quantify the effect of year, month, region,
rainfall and sea surface temperature on CPUE. Biological samples of S. commerson
were collected monthly from within KwaZulu-Natal and southern Mozambique.
Samples were analysed using standard biological techniques. A per-recruit analysis
was conducted using the biological parameters from KwaZulu-Natal and southern
Mozambique in order to assess the status of the S. commerson stock and provide
management recommendations based on the findings. Long-term trends in CPUE
were cyclic in nature with peaks and troughs appearing to be independent of fishing
pressure. Seasonal abundance reflects the south-north migration into KwaZulu-Natal
waters with short term environmental factors such as sea surface temperature
significantly affecting spatial and temporal extent of the migration among regions. S.
commerson spawn in southern Mozambique waters from September to January
(spring-summer) with males maturing at a smaller size (65.2cm FL) compared to
females (82.3cm FL). The overall sex ratio (M: F) was 1:1.36 possibly as a result of
linefishing selecting for faster growing, larger females. S. commerson in KwaZulu-
Natal and southern Mozambique display rapid growth over the first two years before
slowing down considerably after maturity is reached. Females grow faster and live
longer compared to males dominating the older and larger size classes, and attaining
a maximum observed age of 14 years, although fish probably live up to 20 years.
Natural mortality rate was estimated at 0.27 yearˉ¹. Fishing mortality for the combined
region was 0.21 yearˉ¹. The per-recruit analyses for the KwaZulu-Natal and southern
Mozambique indicated that the fishery is being optimally exploited with a current
spawner biomass per recruit at 49% of its theoretical pristine level. Uncertainty with
regards to the fishing pressure in southern Mozambique as a result of illegal fishing
and fishing sectors targeting smaller S. commerson is a cause for concern. The
current recreational daily bag limit of 10 fish.person.dayˉ¹ is considered excessive by
many stakeholders. Given the similarity of the recreational ski-boat fishery in
southern Mozambique, a reduction in the DBL of S. commerson to five fish pppd in
both KwaZulu-Natal and southern Mozambique waters would benefit recreational
fishers by more equitable sharing of the catch and potentially by reducing fishing
mortality at times when the fish are aggregated and vulnerable to high catch rates. A
reduced DBL limit would also reduce the incentive of individual anglers to make large
catches and to sell their fish illegally.
Description
M.Sc. University of KwaZulu-Natal, Durban 2013.
Keywords
Mackerel fisheries., King mackerel--Mozambique., King mackerel--KwaZulu-Natal., Scomberomorus--Mozambique., Scomberomorus--KwaZulu-Natal., Theses--Environmental biology.