Spatio-temporal patterns in maturation of the chokka squid (Loligo vulgaris reynaudii) off the coast of South Africa
- Authors: Olyott, L J H , Sauer, Warwick H H , Booth, Anthony J
- Date: 2006
- Language: English
- Type: Article
- Identifier: vital:6763 , http://hdl.handle.net/10962/d1007930
- Description: Knowledge of the temporal and spatial characteristics of chokka squid (Loligo vulgaris reynaudii) biology in South African waters is limited, so the possibility of there being a geographically fragmented stock was examined by investigating the distribution of maturity patterns for the species, covering all known spawning areas and using both historical and recent data. Gonadosomatic indices (GSI) varied between year-round consistency and apparent seasonal peaks in both summer and winter; there was no clear spatial pattern. Monthly percentage maturity provided further evidence for two peak reproductive periods each year, although mature squid were present throughout. Sex ratios demonstrated great variability between different areas and life history stages. Male-biased sex ratios were only apparent on the inshore spawning grounds and ranged between 1.118:1 and 4.267:1. Size at sexual maturity was also seasonal, squid maturing smaller in winter/spring than in summer/autumn. Also, squid in the east matured smaller than squid in the west. Although the results from the present study do not provide conclusive evidence of distinct geographic populations, squid likely spawn over a significantly larger area of the Agulhas Bank than previously estimated, and squid on the west coast of South Africa may return to spawn on the western portion of the Agulhas Bank. It remains likely, however, that the east and west coast populations are a single stock and that migration of juveniles to the west coast and their subsequent return as sub-adults is an integral but non-essential and variable part of the life history.
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- Date Issued: 2006
Spatial patterns in the biology of the chokka squid, Loligo reynaudii on the Agulhas Bank, South Africa
- Authors: Olyott, L J H , Sauer, Warwick H H , Booth, Anthony J
- Date: 2007
- Language: English
- Type: text , article
- Identifier: http://hdl.handle.net/10962/127053 , vital:35947 , https://doi.10.1007/s11160-006-9027-5
- Description: Although migration patterns for various life history stages of the chokka squid (Loligo reynaudii) have been previously presented, there has been limited comparison of spatial variation in biological parameters. Based on data from research surveys; size ranges of juveniles, subadults and adults on the Agulhas Bank were estimated and presented spatially. The bulk of the results appear to largely support the current acceptance of the life cycle with an annual pattern of squid hatching in the east, migrating westwards to offshore feeding grounds on the Central and Western Agulhas Bank and the west coast and subsequent return migration to the eastern inshore areas to spawn. The number of adult animals in deeper water, particularly in autumn in the central study area probably represents squid spawning in deeper waters and over a greater area than is currently targeted by the fishery. The distribution of life history stages and different feeding areas does not rule out the possibility that discrete populations of L. reynaudii with different biological characteristics inhabit the western and eastern regions of the Agulhas Bank. In this hypothesis, some mixing of the populations does occur but generally squid from the western Agulhas Bank may occur in smaller numbers, grow more slowly and mature at a larger size. Spawning occurs on the western portion of the Agulhas Bank, and juveniles grow and mature on the west coast and the central Agulhas Bank. Future research requirements include the elucidation of the age structure of chokka squid both spatially and temporally, and a comparison of the statolith chemistry and genetic characterization between adults from different spawning areas across the Agulhas Bank.
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- Date Issued: 2007
Spatial description of hake-directed fishing activity off the west coast of South Africa
- Authors: Fairweather, T P , Booth, Anthony J , Sauer, Warwick H H , Leslie, Rob W
- Date: 2010
- Language: English
- Type: text , article
- Identifier: http://hdl.handle.net/10962/127031 , vital:35945 , https://doi.10.2989/18142320609504129
- Description: Historically, the two species of Cape hakes Merluccius capensis and M. paradoxus off South Africa were commercially exploited exclusively by demersal trawling. In 1994, hake-directed demersal longline was introduced on an experimental basis, and in 1998 was initiated as a commercial fishing sector. The effect of a combined fleet composed of both trawlers and longliners on the Cape hake resource is not fully understood. Analysis of fishing intensity and catch-rate data revealed that the highest catch rates were found around the 400m and 500m isobaths for the trawl and longline fisheries respectively. Catch rates from both fishing sectors were also noted to be higher over sediments with a sand component. Differences between areas of the highest fishing intensity and highest catch rates were noted. In addition to other factors, it is suggested that a ‘friction of distance’ effect applies — vessels will trade-off higher catch rates with the increased costs associated with fishing in deeper waters.
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- Date Issued: 2010
Ocean warming affects the distribution and abundance of resident fishes by changing their reproductive scope
- Authors: Potts, Warren M , Booth, Anthony J , Richardson, Timothy J , Sauer, Warwick H H
- Date: 2014
- Language: English
- Type: text , article
- Identifier: http://hdl.handle.net/10962/125352 , vital:35775 , https://doi.10.1007/s11160-013-9329-3
- Description: With ocean warming predicted globally, one of the mechanisms driving distributional shifts and changes in the abundance of resident fishes is reproductive output. The relationship between sea surface temperature and the reproductive activity of a eurythermic, resident coastal species, blacktail seabream Diplodus sargus capensis, was examined in the ‘‘ocean warming’’ hotspot of the northern Benguela. Reproductive activity was found to be restricted to periods when the water temperature dropped below 20 _C. A metadata analysis conducted on the D. sargus sub-species complex similarly showed that reproductive activity was restricted to temperatures between 15 and 20 _C, regardless of the range in ambient water temperature. Based on these findings and using satellite derived SST information, we examined D. s. capensis’s total and seasonal ‘‘reproductive scope’’ that is defined as either the area suitable for spawning each year or the duration of its potential spawning season at a fixed geographical locality, respectively. Trends were examined over the last three decades. Reproductive scope by area was found to be shrinking at a rate of 7 % per decade in southern Angola and expanding at a rate of 6 % per decade in northern Namibia. Reproductive scope by season decreased by 1.05 months per decade in Namibe, southern Angola and increased by 0.76 months per decade in Hentiesbaai, northern Namibia. Changes in reproductive scope may be a driving mechanism of distributional shifts in resident fishes, although the rate of the shifts is likely to be slow. More importantly, changes in reproductive scope will not be uniform throughout fish distributions and will most likely result in heterogeneous variations in fish abundance.
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- Date Issued: 2014
Ocean warming, a rapid distributional shift, and the hybridization of a coastal fish species
- Authors: Potts, Warren M , Henriques, Romina , Santos, Carmen V D , Munnik, Kate , Ansorge, Isabelle J , Dufois, Francois , Sauer, Warwick H H , Booth, Anthony J , Kirchner, Carola , Sauer, Warwick , Shaw, Paul W
- Date: 2014
- Language: English
- Type: text , article
- Identifier: http://hdl.handle.net/10962/125375 , vital:35777 , https://doi.10.1111/gcb.12612
- Description: Despite increasing awareness of large-scale climate-driven distribution shifts in the marine environment, no study has linked rapid ocean warming to a shift in distribution and consequent hybridization of a marine fish species. This study describes rapid warming (0.8 °C per decade) in the coastal waters of the Angola-Benguela Frontal Zone over the last three decades and a concomitant shift by a temperature sensitive coastal fish species (Argyrosomus coronus) southward from Angola into Namibia. In this context, rapid shifts in distribution across Economic Exclusive Zones will complicate the management of fishes, particularly when there is a lack of congruence in the fisheries policy between nations. Evidence for recent hybridization between A. coronus and a congener, A. inodorus, indicate that the rapid shift in distribution of A. coronus has placed adults of the two species in contact during their spawning events. Ocean warming may therefore revert established species isolation mechanisms and alter the evolutionary history of fishes. While the consequences of the hybridization on the production of the resource remain unclear, this will most likely introduce additional layers of complexity to their management.
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- Date Issued: 2014