Aspects of the biology and fisheries of an economically important sparid Dentex macrophthalmus (Bloch 1791) in the Namibe province, Angola
- Authors: Potts, Warren M , Inácio, L A , Santos, Carmen V D , Richardson, Timothy J , Sauer, Warwick H H
- Date: 2010
- Language: English
- Type: text , article
- Identifier: http://hdl.handle.net/10962/123096 , vital:35405 , https://doi.org/10.2989/1814232X.2010.538160
- Description: Dentex macrophthalmus is a small, commercially important sparid fish that is abundant in the deeper waters (range 50–500 m) of the Mediterranean Sea (Trunov 1970), the North-West African shelf (Goode and Bean 1896, Fowler 1936), including the Cape Verde and Canary Islands, and the south-eastern Atlantic from the Equator to just south of Lüderitz (27°40′ S) (Trunov 1970). Because of its high abundance, it is of considerable economic importance and is captured in large numbers in both artisanal and commercial fisheries in many countries. Although it is the most important artisanal species along the entire Angolan coast, it is most abundant in the southern region (Kilongo et al. 2007). Despite its economic importance in Angola and elsewhere, relatively little is known on its abundance, fisheries and biology.
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- Date Issued: 2010
Behavioural observations of the common octopus Octopus vulgaris in Baía dos Tigres, southern Angola
- Authors: de Beer, Chénelle L , Potts, Warren M
- Date: 2013
- Language: English
- Type: text , article
- Identifier: http://hdl.handle.net/10962/123233 , vital:35419 , https://doi.org/10.2989/1814232X.2013.847496
- Description: The common octopus Octopus vulgaris (Cuvier 1797) is a shallow-water cephalopod species that inhabits coastal rocky areas or reefs. It is the most studied and widely distributed species in its genus. This species has a short lifespan and a rapid growth rate of over 5% of body weight per day (García and Valverde 2006, Ibáñez and Keyl 2010). It is semelparous, with females laying large strings of eggs that they attach to the substrata in their dens (Hernández- García et al. 2002). There is a paucity of information on the preferred habitat and behaviour of O. vulgaris in its natural environment (Anderson 1997, Meisel et al. 2006). The species has been classified as highly mobile, yet resident individuals may exhibit high levels of territoriality. Its mobility allows it to move efficiently between a wide range of suitable habitats (Katsanevakis and Verriopoulos 2004). Octopus make use of both tactile and visual senses to forage, feed and seek suitable shelter (Forsythe and Hanlon 1997, Carvalho and Sousa Reis 2003, Rodríguez-Rúa et al. 2005). They are active feeders that seek out prey such as crustaceans, sessile molluscs and small fish (Forsythe and Hanlon 1997, Boyle and Rodhouse 2005), as well as their smaller conspecifics (Ibáñez and Keyl 2010). The majority of behavioural ecology studies on octopus have been on captive individuals. However, the activity patterns (Mather 1988), foraging strategies and aspects of the movement behaviour (Mather and O’Dor 1991) of juvenile O. vulgaris in the wild were described for an exploited population in Bermuda. Because intertidal octopus stocks are exploited by recreational, subsistence and artisanal fisheries (e.g. Oosthuizen and Smale 2003, Sauer et al. 2011), there are few opportunities to study the behavioural ecology of unexploited populations. An opportunity arose to study various behavioural patterns in an unexploited population in Baía dos Tigres, a large (~200 km2) coastal embayment situated on an isolated stretch of the southern Angolan coast (Figure 1). This study provides information on the population size structure, aspects of the movement behaviour, activity patterns and foraging strategies of O. vulgaris in this largely unstudied coastal embayment.
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- Date Issued: 2013
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