Marine and Freshwater Research Marine and Freshwater Research Society
Advances in the aquatic sciences
RESEARCH ARTICLE

Population structure and biology of shortfin mako, Isurus oxyrinchus, in the south-west Indian Ocean

J. C. Groeneveld A F , G. Cliff B C , S. F. J. Dudley D , A. J. Foulis A , J. Santos E and S. P. Wintner B C

A Oceanographic Research Institute, PO Box 10712, Marine Parade 4056, Durban, South Africa.

B KwaZulu-Natal Sharks Board, Private Bag 2, Umhlanga Rocks 4320, South Africa.

C Biomedical Resource Unit, University of KwaZulu-Natal, PO Box X54001, Durban 4000, South Africa.

D Fisheries Management, Department of Agriculture, Forestry and Fisheries, Private Bag X2, Rogge Bay 8012, South Africa.

E Norwegian College of Fishery Science, University of Tromsø, NO-9037, Tromsø, Norway.

F Corresponding author. Email: jgroeneveld@ori.org.za

Marine and Freshwater Research 65(12) 1045-1058 http://dx.doi.org/10.1071/MF13341
Submitted: 23 December 2013  Accepted: 1 May 2014   Published: 24 September 2014

Abstract

The population structure, reproductive biology, age and growth, and diet of shortfin makos caught by pelagic longliners (2005–10) and bather protection nets (1978–2010) in the south-west Indian Ocean were investigated. The mean fork length (FL) of makos measured by observers on longliners targeting tuna, swordfish and sharks was similar, and decreased from east to west, with the smallest individuals occurring near the Agulhas Bank edge, June to November. Nearly all makos caught by longliners were immature, with equal sex ratio. Makos caught by bather protection nets were significantly larger, males were more frequent, and 93% of males and 55% of females were mature. Age was assessed from band counts of sectioned vertebrae, and a von Bertalanffy growth model fitted to sex-pooled length-at-age data predicted a birth size (L0) of 90 cm, maximum FL (L) of 285 cm and growth coefficient (k) of 0.113 y–1. Males matured at 190 cm FL, aged 7 years, and females at 250 cm, aged 15 years. Litter sizes ranged from nine to 14 pups, and the presence of gravid females in bather protection nets suggested that some pupping occurred in shelf waters. Teleosts (mainly Trachurus capensis) occurred in 84% of stomachs collected on longliners, whereas elasmobranchs (63.5%) were most common in samples collected from bather protection nets, followed by teleosts (43.1%) and cephalopods (36.5%). Larger prey size may be a factor that attracts large makos to coastal waters.

Additional keywords: demography, GLM, pelagic longline fisheries, shark bycatch, stomach contents.


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