Spatial description of hake-directed fishing activity off the west coast of South Africa
- Fairweather, T P, Booth, Anthony J, Sauer, Warwick H H, Leslie, Rob W
- 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.
- Full Text:
- Date Issued: 2010
- 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.
- Full Text:
- Date Issued: 2010
Determining the daily spawning cycle of the chokka squid, Loligo reynaudii off the South African Coast
- Authors: Melo, Y , Sauer, Warwick H H
- Date: 2007
- Language: English
- Type: text , article
- Identifier: http://hdl.handle.net/10962/124275 , vital:35588 , https://doi.10.1007/s11160-006-9034-6
- Description: The study investigated the duration and peak of the daily spawning cycle of the chokka squid Loligo reynaudii, and the possibility of estimating the spawning frequency by means of classification and grouping (by age) of postovulatory follicles. Post-ovulatory follicles were classified into three types: new, intermediate or old, based in part on the degree of resorption of the follicles determined from histological sections. Ovulation in this species appears to be a rapid process, seen by the occurrence of both mature oocytes and post-ovulatory follicles in partially spent ovaries. Results suggest that chokka squid present a diel timing of spawning, with successive periods of egg accumulation at night, interrupted by periods of active spawning during the day. The ovarian cycle for this species appears typical of serial spawners. During the spawning phase ovaries go through ripe, partially spent and recovering stages by undergoing a process of maturation, ovulation and redeveloping, where a new batch of advanced oocytes is recruited. The cycle typically appears to last between 24 and 36 h, and may depend on a number of factors such as environmental conditions and the presence of predators. The short-lived (about 14 h) nature of the post-ovulatory follicles precludes the calculation of the exact spawning interval and does not allow us to accurately predict the spawning frequency. Together with the degree of oviduct fullness they do however give a good indication of imminent or recent spawning.
- Full Text:
- Date Issued: 2007
- Authors: Melo, Y , Sauer, Warwick H H
- Date: 2007
- Language: English
- Type: text , article
- Identifier: http://hdl.handle.net/10962/124275 , vital:35588 , https://doi.10.1007/s11160-006-9034-6
- Description: The study investigated the duration and peak of the daily spawning cycle of the chokka squid Loligo reynaudii, and the possibility of estimating the spawning frequency by means of classification and grouping (by age) of postovulatory follicles. Post-ovulatory follicles were classified into three types: new, intermediate or old, based in part on the degree of resorption of the follicles determined from histological sections. Ovulation in this species appears to be a rapid process, seen by the occurrence of both mature oocytes and post-ovulatory follicles in partially spent ovaries. Results suggest that chokka squid present a diel timing of spawning, with successive periods of egg accumulation at night, interrupted by periods of active spawning during the day. The ovarian cycle for this species appears typical of serial spawners. During the spawning phase ovaries go through ripe, partially spent and recovering stages by undergoing a process of maturation, ovulation and redeveloping, where a new batch of advanced oocytes is recruited. The cycle typically appears to last between 24 and 36 h, and may depend on a number of factors such as environmental conditions and the presence of predators. The short-lived (about 14 h) nature of the post-ovulatory follicles precludes the calculation of the exact spawning interval and does not allow us to accurately predict the spawning frequency. Together with the degree of oviduct fullness they do however give a good indication of imminent or recent spawning.
- Full Text:
- Date Issued: 2007
Managing South Africa's trawl bycatch
- Walmsley, Sarah A, Leslie, Rob W, Sauer, Warwick H H
- Authors: Walmsley, Sarah A , Leslie, Rob W , Sauer, Warwick H H
- Date: 2006
- Language: English
- Type: text , article
- Identifier: http://hdl.handle.net/10962/124954 , vital:35713 , https://doi.10.1093/icesjms/fsl022
- Description: Over the past few decades, it has become widely recognized that the management strategies of world fisheries must ensure sustainability of bycatch as well as of target species. South Africa implemented a pilot observer programme from 1995 to 2000 to collect data on the levels and patterns of bycatch and discards in the demersal trawl fishery. Here, the results of that programme are used in conjunction with information on bycatch value and compliance to assess the issues and problems regarding bycatch and discarding in the fishery. In general, bycatch components can be placed into one of three categories (discarded bycatch, retained bycatch, and processing waste), each of which present different management problems and require different management approaches. The results were used to formulate a bycatch management plan for the demersal trawl fishery in South Africa. Given the need to continue monitoring bycatch, the performance of the pilot observer programme is reviewed, and the levels of sampling effort required for a national programme are discussed.
- Full Text:
- Date Issued: 2006
- Authors: Walmsley, Sarah A , Leslie, Rob W , Sauer, Warwick H H
- Date: 2006
- Language: English
- Type: text , article
- Identifier: http://hdl.handle.net/10962/124954 , vital:35713 , https://doi.10.1093/icesjms/fsl022
- Description: Over the past few decades, it has become widely recognized that the management strategies of world fisheries must ensure sustainability of bycatch as well as of target species. South Africa implemented a pilot observer programme from 1995 to 2000 to collect data on the levels and patterns of bycatch and discards in the demersal trawl fishery. Here, the results of that programme are used in conjunction with information on bycatch value and compliance to assess the issues and problems regarding bycatch and discarding in the fishery. In general, bycatch components can be placed into one of three categories (discarded bycatch, retained bycatch, and processing waste), each of which present different management problems and require different management approaches. The results were used to formulate a bycatch management plan for the demersal trawl fishery in South Africa. Given the need to continue monitoring bycatch, the performance of the pilot observer programme is reviewed, and the levels of sampling effort required for a national programme are discussed.
- Full Text:
- Date Issued: 2006
Spatio-temporal patterns in maturation of the chokka squid (Loligo vulgaris reynaudii) off the coast of South Africa
- Olyott, L J H, Sauer, Warwick H H, Booth, Anthony J
- 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.
- Full Text:
- Date Issued: 2006
- 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.
- Full Text:
- Date Issued: 2006
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