Bycatch and discarding in the South African demersal trawl fishery
- Walmsley, Sarah A, Leslie, Rob W, Sauer, Warwick H H
- Authors: Walmsley, Sarah A , Leslie, Rob W , Sauer, Warwick H H
- Date: 2007
- Language: English
- Type: text , article
- Identifier: http://hdl.handle.net/10962/123944 , vital:35518 , https://doi.10.1016/j.fishres.2007.03.002
- Description: Observers aboard commercial trawlers collected data on the total catch composition of 614 and 479 hauls made by vessels operating off the south and west coasts of South Africa, respectively. On the south coast, four fishing areas were identified on the basis of target species and fishing depth. On the west coast, hauls were separated into those targeting hake Merluccius spp. in four depth ranges (0–300, 301–400, 401–500, and >500 m) and those targeting monkfish Lophius vomerinus. For each area, the catch composition was calculated and the species assemblages were investigated using cluster analysis and multi-dimensional scaling. Finally, for each coast, the weight of fish discarded annually was estimated. On the south coast, although hake dominated, between 21% and 47% of the catch was not hake, depending on the fishing area. In comparison, hake dominated west coast catches, the proportion of hake increasing with depth. For each fishery investigated, approximately 90% of the catch was processed and landed. However, estimates of annual discards indicate that the south and west coast fisheries may annually discard 9000 or 10,000 t and 17,000 or 25,000 t, of undersized and unutilizable fish and offal, respectively, depending on the estimation method used. When developing strategies to limit or enhance utilization of bycatch, cognisance should be taken of the differences in catch composition between the south and west coasts and of the importance of bycatch revenue to south coast fishing companies.
- Full Text:
- Date Issued: 2007
- Authors: Walmsley, Sarah A , Leslie, Rob W , Sauer, Warwick H H
- Date: 2007
- Language: English
- Type: text , article
- Identifier: http://hdl.handle.net/10962/123944 , vital:35518 , https://doi.10.1016/j.fishres.2007.03.002
- Description: Observers aboard commercial trawlers collected data on the total catch composition of 614 and 479 hauls made by vessels operating off the south and west coasts of South Africa, respectively. On the south coast, four fishing areas were identified on the basis of target species and fishing depth. On the west coast, hauls were separated into those targeting hake Merluccius spp. in four depth ranges (0–300, 301–400, 401–500, and >500 m) and those targeting monkfish Lophius vomerinus. For each area, the catch composition was calculated and the species assemblages were investigated using cluster analysis and multi-dimensional scaling. Finally, for each coast, the weight of fish discarded annually was estimated. On the south coast, although hake dominated, between 21% and 47% of the catch was not hake, depending on the fishing area. In comparison, hake dominated west coast catches, the proportion of hake increasing with depth. For each fishery investigated, approximately 90% of the catch was processed and landed. However, estimates of annual discards indicate that the south and west coast fisheries may annually discard 9000 or 10,000 t and 17,000 or 25,000 t, of undersized and unutilizable fish and offal, respectively, depending on the estimation method used. When developing strategies to limit or enhance utilization of bycatch, cognisance should be taken of the differences in catch composition between the south and west coasts and of the importance of bycatch revenue to south coast fishing companies.
- Full Text:
- Date Issued: 2007
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
Spatial patterns in the biology of the chokka squid, Loligo reynaudii on the Agulhas Bank, 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: 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.
- Full Text:
- Date Issued: 2007
- 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.
- Full Text:
- Date Issued: 2007
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