Chokka Research Articles

Can the Loligo squid fishery be managed effectively? A synthesis of research on Loligo vulgaris reynaudii

The question whether the chokka squid Loligo vulgaris reynaudii resource can be managed effectively is examined in the light of developments in the fishery and knowledge of the resource gathered in recent years. The history of the fishery since its inception in 1985, management controls and the current state of knowledge of stock identity, distribution, seasonality and abundance, life cycle and population dynamics are reviewed as a background to a discussion of management alternatives. Long-term measures, such as fleet limitation, seasonal closures and de facto closed areas and size limits, all appropriate in the early stages of a fishery when knowledge is limited, should be replaced by more-flexible alternatives as knowledge improves. Methods of stock assessment, stock-recruitment studies and further basic biological analyses are recognized as being crucial to improving management methods. An explanation embracing physical oceanographic features and plankton productivity in the vicinity of the spawning grounds is advanced for the mechanisms underlying hatchling survival. The possible contribution of trophic studies to squid management is recognized. Effective management of the chokka squid resource is concluded to be possible provided that the specific areas needing attention be addressed by means of goal-orientated research. Rational, but not necessarily optimal, management decisions can, however, be made even in the absence of stock-recruitment data, by the employment of common-sense measures which generally include some form of effort control.

A note on the stomach contents of fur sealsArctocephalus pusillus pusillusbeached on the south-east coast of South Africa

The stomach contents of 36 South African fur seals beached over a 20 year period along the south-east coast of South Africa were examined. In all, 15 fish and nine cephalopod prey types were identified. Five species of fish (Merluccius capensis, Trachurus trachurus capensis, Cynoglossus zanzibarensis, Pterogymnus laniarius, Austroglossus pectoralis) and two cephalopods (Loligo vulgaris reynaudii, Octopus sp.) were of major importance in the diet. The proportion of demersal species in the diet is higher than previously reported, suggesting that the seals forage extensively on the sea floor. The most important fish species in the diet was M. capensis, and the most important cephalopod was L. v. reynaudii. Male and female seals showed distinct prey selection, suggesting possible intraspecific resource partitioning. There is no evidence of competition between seals and other top predators.

A preliminary investigation of the effect of sea temperature fluctuations and wind direction on catches of chokka squidLoligo vulgaris reynaudiioff the Eastern Cape, South Africa

Opportunistically collected information on sea temperature and wind was used in a preliminary investigation of physical factors affecting jigged catches of chokka squid Loligo vulgaris reynaudii. Results revealed sea temperature and wind to be correlated with catches during part of the summer upwelling season (October and December) in 1988. Multiple linear regression analysis confirmed that sea temperature was a highly significant explanatory variable (p = 0,0000) for catches from three boats, as well as for the overall catch. Wind direction also played a significant role (p = 0,0085) in the overall catch. A statistical linear multiple regression model is proposed for each boat and for the total catch. Upwelling events are suggested to play a major role in the availability of squid on the inshore spawning grounds (<50 m) during summer. With further study, other independent physical variables, such as currents, swell height and turbidity, are likely to be introduced into the proposed model.

Food of squidLoligo vulgaris reynaudii(Cephalopoda: Loliginidae) on their spawning grounds off the Eastern Cape, South Africa

Squid on their spawning grounds feed mainly at night when they are more dispersed. In all, 28,5 per cent of squid sampled at night had food in their stomachs, whereas only 6,7 per cent of those collected by day had food. Most stomachs contained only a single prey species (90,8% of those analysed). Teleosts dominated the prey of squid collected at night (58%M), followed by crustaceans (25%), polychaetes (9%) and cephalopods (3%). By day the prey was dominated by cephalopods (61%), followed by teleosts (20%), crustaceans (14%) and polychaetes (2%). Bregmaceros sp. was the most frequently identified teleost, Loligo vulgaris reynaudii (cannibalism) the dominant cephalopod, ?Betaeus sp. (Alpheidae; pieces of coral were also found) the commonest crustacean and Nereis sp. the commonest polychaete. There were seasonal differences in the diet too: squid caught during summer seemed to have similar proportions of the four main prey components. Also, the incidence of feeding was higher than in winter. There were, however, no significant differences in stomach mass expressed as a percentage of body mass between day and night nor between season nor between sexes. The average stomach mass of all samples was 0,6 per cent of body mass, less than on the feeding grounds, where it is generally about 2 per cent.

The biomass and ecology of chokka squidLoligo vulgaris reynaudiioff the West Coast of South Africa

Migration, stock size and ecology of chokka squid Loligo vulgaris reynaudii off the West Coast of South Africa were studied and their relationship to other regions compared by analysis of distributional, biomass, and size composition, and biological data collected from biannual research cruises from 1983-1987. Biomass was lower than on the South Coast, higher in summer than in winter and declined over the duration of the study. The distribution of squid was generally continuous from the east up to between Cape Point and Cape Columbine. North of Cape Columbine it was more sporadic and occurred only occasionally in the far north (north of 31°S). Depth distribution, at up to 350 m, was much greater than on the S. Coast where an insignificant percentage of the biomass occurs over 200 m. The size composition was unimodal compared to a polymodal distribution on the S. Coast, and mean and modal sizes were smaller. Maturity rates and gonado-somatic indices were also much lower. The percentage of feeding squid was...

Biological studies on the chokker squidloligo vulgaris reynaudii(Cephalopoda; Myopsida) on spawning grounds off the south-east coast of South Africa

The biology of the chokker squid Loligo vulgaris reynaudii on inshore grounds in the South-East Cape was studied by examination of trends in jig catches, length frequency analyses, length/weight relationships, analyses of maturity stages and SCUBA diving transects in a spawning area. Commercial catches probably reflected the influx of squid into shallow waters to spawn. Modal length at spawning was between 265 and 305 mm mantle length (ML) in males and between 175 and 195 mm ML in females. There was no modal progression to indicate growth, nearly all squid were mature, a percentage of males were spent, the level of feeding on the grounds was very low, and mantle walls were thinner than those of squid offshore. These factors all indicate that the squid population inshore had reached the end of its life cycle and was in the spawning phase. The sex ratio was biased towards males, an indication either of differential inshore migration or of greater longevity in males than in females. The frequency of occurren...

Food and feeding of Loligo vulgaris reynaudii from St Francis Bay, South Africa

Change of colour, consistency and pH in contents of the caecum and the stomach of laboratory-maintained long-finned squid Loligo vulgaris reynaudii were determined. There was relatively little variability in any of these parameters of the specimens investigated. Colour and consistency of food or emulsion were used as a basis of analysis of stomach-caecum contents of wild squid. Most squid fed late during the night and/or during the early morning, and the frequency of caecum colour categories did not change much between trawls. Wild squid preyed upon different organisms according to their size. Squid of 69–125 mm dorsal mantle length fed mainly on euphausiids (95% by frequency of occurrence, 87,5% by mass) and those of 126–240 mm mainly on fish (78 and 74,3% respectively) with Bregmaceros ?macclellandii and hake as important components. Unidentified fish in the stomachs (i.e. those from which no otoliths were available) probably also belonged to these two species.

The feeding habits of six pelagic and predatory teleosts in eastern Cape coastal waters (South Africa)

A four‐year study of the feeding habits of six pelagic teleost species in eastern Cape coastal waters of South Africa is analysed. These predators are Lichia amia, Pomatomus sallatrix, Seriola lalandi, Atractoscion aequidens, Katsuwonus pelamis and Thunnus albacares. All species occur in inshore coastal water (&lt; 1 ‐8 km from shore) for at least part of the year. The tunas were also sampled from the edge of the continental shelf (40–50 km from shore). The use of squid ‘beaks' and fish otoliths allowed accurate prey identification and size estimates. Variations in prey taken are attributed to differences in locality, predator species and size. Three pelagic fish species are important to all the predators: Sardinops ocettata, Etrumeus teres and Engraulis capensis. The inshore squid, Loligo reynaudi, is another major prey item. Offshore tuna samples revealed a different prey spectrum, viz. oceanic squid and saury, Scomberesox saurus, being taken frequently.

Electrophoresis in the study of diets and digestive rates of seabirds

1. 1. Attempts were made to identify unknown gut contents of seabirds by protein analysis using electrophoresis. 2. 2. Standards of undigested fish and squid muscle tissue were compared with muscle tissue at various stages of digestion. 3. 3. Digested mixtures of squid ( Loligo reynaudi), Pelagic Goby ( Sufflogobius bibarbatus) and Cape Anchovy ( Engraulis capensis) did not resemble the undigested standards of each species respectively. 4. 4. Electrophoresis could prove useful in the study of differential digestion rates of seabird prey species.

Growth of Jackass penguin chicks (Spheniscus demersus) hand reared on different diets

Jackass penguin chicks from the age of about 10 days were hand reared on different diets: mullet Liza richardsoni, anchovy Engraulis capensis and squid Loligo reynaudi, until they fledged. The weight of food fed and the daily weight increment of the chicks was measured. Excreta were collected every 5–6 days and analysed to establish metabolized energy and metabolic efficiency. Culmen measurements were taken regularly and plotted against age. The age, weight and sum of food fed up to fledging was compared between diets as well as with field data. Compensatory growth was exhibited when stunted chicks on a relatively poor diet had their diet changed to a relatively better diet.

Prédation and prey selectivity byArgyrosomus hololepidotus(Osteichthyes: Sciaenidae) in south-eastern Cape waters of South Africa

Argyrosomus hololepidotus (kob) is a top predator in the shallow marine environment of the south-eastern Cape. There is considerable change in prey taken over the size range of predators examined (256-1701 mm). Young A. hololepidotus are planktivorous taking swarming mysids, but as they grow they become piscivorous, feeding on both pelagic and demersal prey associated with soft sediments. The pelagic cephalopod Loligo reynaudi is also taken by large specimens. Prey selection varies with locality and time of year. A field experiment in which the prey were compared with fish caught in a small-mesh net revealed a preference for some slow-moving demersal species. The length distributions of dominant prey species are given and these clearly show that larger prey are preferred by larger predators.

Species diversity and size ranges of cephalopods in the diet of jackass penguins from Algoa Bay, South Africa

An analysis of cephalopod remains collected from jackass penguin Spheniscus demersus regurgitations and stomachs at St Croix Island, Algoa Bay, in 1976 and 1977 revealed the presence of three cephalopod species. Most common was Loligo reynaudi, with Heterotuthus sp. and Argonauta argo constituting trace items. Beak measurements were used to estimate dorsal mantle lengths and wet masses of cephalopods taken by penguins. The dominant cephalopod taken by penguins was the same species as that harvested by commercial fisheries. Despite an overlap in the size range of squid taken by penguins and trawlers, penguins tended to take smaller squid and did not take the larger squid landed by trawlers. The seasonal occurrence of squid in the penguin diet in 1976 and 1977 was the opposite of the abundance of squid in the area as reflected by trawler landings. This is explained in terms of the abundance and preference for other prey items. It is suggested that penguins take fish in preference to squid because of the rel...

Quantification and representation of potential spatial interactions in the southern Benguela ecosystem

This work explores the potential spatial interactions between 13 key commercial species of the southern Benguela ecosystem: sardine Sardinops sagax, anchovy Engraulis encrasicolus, round herring Etrumeus whiteheadi, horse mackerel Trachurus trachurus capensis, chub mackerel Scomber japonicus, chokka squid Loligo vulgaris reynaudii, kingklip Genypterus capensis, Cape hake Merluccius spp., silver kob Argyrosomus inodorus, snoek Thyrsites atun, albacore Thunnus alalunga, bigeye tuna Thunnus obesus and yellowfin tuna Thunnus albacares. It is based on distribution maps per species after combining available commercial and research databases. The resulting 78 pairs of potential interactions are quantified using three indices: the overlap in area, the overlap in biomass and the weighted kappa index. From additional information on the diet of the different species and trophic models, the main trophic interactions (predation or competition) were identified and mapped. The results are discussed with regard to methodological limitations, habitat selection, fish assemblages, the need for spatial resolution of trophic models and the ecosystem approach to fishery management.

Interpreting behavioural data from Radio-Acoustic Positioning Telemetry (RAPT) systems

To detect behavioural patterns of individually tagged squid Loligo vulgaris reynaudii in a Radio-Acoustic Positioning Telemetry (RAPT) buoy array, trajectories reflecting the four dimensions of latitude, longitude, depth and time were plotted from data collected during field experiments in South Africa. Finding a continuous curve to represent the sampled trajectories required dealing with anisotropic precision and accuracy, non-uniform sampling rates and improbable outliers. A combination of an operator-controlled smoothing option of an approximating cubic spline and a weight factor assignment based on distance from the mass curve gave the most expedient results when compared with video recordings.