Biological characteristics of sardine caught by the beach-seine fishery during the KwaZulu-Natal sardine run

Abstract

Catches of sardine Sardinops sagax made by the beach-seine fishery off the KwaZulu-Natal (KZN) coast during the annual winter sardine run have been sampled intermittently since 1951, and a substantial amount of data on various biological characteristics of sardine caught off KZN now exists. Information on length frequency distributions, mean caudal length (CL), length-mass relationships, condition factor (CF), age distributions, sex ratio, gonad maturity and diet have been collated and are presented in this paper. Sardine caught off the KZN coast during the past six decades had a mean CL of 17.4 cm (SD 2.0) and have shown a significant increasing trend in mean CL through time; had substantially lower mass-at-length compared to fish that did not participate in the sardine run; have shown two distinct periods of decrease in CF between 1979 and 2005; ranged in age from 1 to 7 years but were numerically dominated by younger fish (1-, 2-or 3-year-olds); had a 1:1 sex ratio and gonads that were predominantly actively developing and undergoing maturation and vitellogenesis; and had a diet dominated by zooplankton, primarily calanoid and harpactacoid copepods and fish eggs. These characteristics, together with some information on meristic and morphological characteristics of sardine caught off KZN, are compared to characteristics of sardine from elsewhere off South Africa's coast to assess the possibility that the former may be a distinct stock or a functionally discrete adult assemblage. Significant differences in characteristics such as CF, vertebral count and body shape were found, supporting the hypothesis that KZN sardine represent a distinct stock or a functionally discrete adult assemblage. However, small sample sizes and plausible alternative explanations for these differences weaken, but do not invalidate, this hypothesis, and further work is needed to answer this question.

Incorporating parasite data in population structure studies of South African sardine Sardinops sagax

“…Published and unpublished information on biological characteristics of sardine from the west and south coast collected from commercial catch and research survey samples was collated by van der Lingen et al . (2009) in order to examine whether spatial variability in these characteristics occurred and, if so, whether such provided plausible evidence for the existence of two stocks. Sardine condition factor (calculated as the ratio of observed to expected wet body mass) showed significant spatial variation, with fish from east of Cape Agulhas showing higher condition factor values than those from the west coast (van der Lingen et al .…”

“…Subsequent analyses of vertebral count and body shape data from sardine collected from South Africa's east coast showed that fish from this region had significantly fewer vertebrae and a different body shape compared with those from the west or south coasts (van der Lingen et al . 2010 b ).…”

“…A consistent difference in annual length-at-50% sexual maturity of female sardine has also been documented, with fish off the south coast maturing at a larger size than those off the west coast (van der Lingen et al . 2009). Mean size-at-age and otolith length-at-age were not significantly different for sardine from the South African west and south coasts, although in both cases there was a slight tendency for values to be higher for fish from the east of Cape Agulhas (van der Lingen et al .…”

“…Mean size-at-age and otolith length-at-age were not significantly different for sardine from the South African west and south coasts, although in both cases there was a slight tendency for values to be higher for fish from the east of Cape Agulhas (van der Lingen et al . 2009).…”

“…These significant differences in most of the sardine phenotypic characteristics studied were taken as support for the hypothesis of multiple sardine stocks off South Africa (van der Lingen et al . 2009).…”