Seasonality of beach-seine catches in False Bay, South Africa, and implications for management

Abstract

Commercial beach-seine catches and effort in False Bay are strongly seasonal, 93% of the to~al catch being taken in spring, summer and autumn. Catch per haul of the target species Liza richardsonii in May (autumn) and October (spring) was double that recorded in summer, even though effort was only half of that during summer. Effort was largely dependent on the frequency of occurrence of patches of the surf-zone diatom Anaulus birostratus, on which L. richardsonii feed and around which the nets were set. Species important in recreational anglers' catches, namely Lithognathus lithognathus, Diplodus sargus capensis, Rhabdosargus globiceps, Seriola lalandi, Dichistius capensis, Pomatomus saltatrix, Umbrina canariensis and Argyrosomus hololepidotus, were also more abundant in spring, summer and autumn. It is concluded that the present winter closed season, during which "angling" fish have to be released, is not having the desired effect of reducing the total catch of these species because they are more abundant in summer. A possible alternative to the present partially closed season would be to disallow seine-netting on weekends and public holidays. This may reduce the total catch of "angling" species by 30%.Handelsvangste en -poging met strandseens in Valsbaai wissel seisoensgewys sterk en 93% van die algehele vangs word in die lente, somer en herfs ingebring. Vangs per trek van die teikenspesie Liza richardsonii was in Mei (herfs) en Oktober (Iente) dubbel wat dit in die somer was, selfs ofskoon die poging maar die helfte was van wat dit in die somer was. Poging hang graotliks af van die voorkomsfrekwensie van kolle van die brandingsonediatoom Anaulus birostratus wat L. richardsonii bewei en waar random die nette gegooi is. Spesies wat belangrik in die vangste van ontspanningshengelaars is, te wete Lithognathus lithognathus, Diplodus sargus capensis, Rhabdosargus globiceps, Seriola lalandi, Dichistius eapensis, Pomatomus saltatrix, Umbrina eanariensis en Argyrosomus hololepidotus, was ook in die lente, somer en herfs volopper. Die gevolgtrekking word gemaaak dat die huidige geslote winterseisoen waartydens "hengel"-vis vrygelaat moet word, nie die gewenste uitwerking het om die totale vangs van hierdie spesies te verminder nie omdat hulle in die somer vol opper is. 'n Moontlike altematief vir die huidige dee Is geslote seisoen sou wees om seennetvangs oor naweke en openbare vakansiedae te belet. Dit kan die totale vangs van "hengel"-spesies met 30% verminder.

Residency, Habitat Use and Sexual Segregation of White Sharks, Carcharodon carcharias in False Bay, South Africa

“…While we have no data for social interactions between white sharks in False Bay there is information available on both thermal and food variables within the Bay that may help explain the marked seasonal patterns in aggregation sites. Water temperature within the Bay is highest in spring and summer and is associated with inshore diatom blooms, which promote spawning and recruitment by a diverse assemblage of fish [53], [37], [38] in addition to higher fish abundance [37], [38]. Given the close association between prey abundance and shark distribution [54], [55], [56]; [57], [58] it is possible that the combination of increased difficulty in catching juvenile seals at the island, with concomitant increase in the availability of a variety of fish species in the Inshore region, may explain the marked seasonal shift in white shark habitat use within the Bay.…”

“…The coastline of False Bay forms part of the City of Cape Town metropole, which has a population of approximately 3.8 million people. The inshore region of the Bay is characterized by a broad range of habitats, including reef, sand and mixed reef and sand and supports a rich diversity of both teleosts and elasmobranchs [37], [38]. A single island (Seal Island) is located within the northern section of the Bay and is home to the second largest island-based breeding colony of Cape fur seals in South Africa (unpublished data).…”

The Influence of Environmental Variables on the Presence of White Sharks, Carcharodon carcharias at Two Popular Cape Town Bathing Beaches: A Generalized Additive Mixed Model

“…Within the spring and summer period, temporary upwelling events result in colder waters along the eastern and middle areas of False Bay and warmer waters in the shallower, especially northern areas of the Bay, which include Muizenberg and Fish Hoek [26]. The warmer water results in blooms of surf-zone diatoms, which are associated with an increase in abundance and diversity of teleosts and chondrichthyans [25], [27], which in turn are prey for white sharks [13], [28], [29]. These upwelling events are also responsible for an increase in beach-seine catches of teleosts and chondrichthyans along this stretch of coast [30], [31].…”

Regional variation in the life history of the canary drum Umbrina canariensis (Sciaenidae), in South African waters

“…It is an important by-catch component in demersal trawl fisheries operating off the West African coast (Podosinnikov, 1979;Venidictova, 1982Venidictova, , 1983a and off the South African south coast (Japp et al, 1994;Smale and . Lamberth et al, 1994Lamberth et al, , 1995 and shore anglers (e.g. Bennett, 1991;Bennett and Attwood, 1993; undoubtedly refer to U. robinsoni .…”