Branch Fisheries Management

Cape Town, South Africa

Branch Fisheries Management

Cape Town, South Africa

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Cohen L.A.,University of Cape Town | Pichegru L.,Nelson Mandela Metropolitan University | Gremillet D.,CNRS Center of Evolutionary and Functional Ecology | Coetzee J.,Branch Fisheries Management | And 2 more authors.
Marine Ecology Progress Series | Year: 2014

Seabirds respond to environmental changes by adjusting their breeding and foraging strategies, but this behavioural flexibility has limits. Cape gannets Morus capensis breeding in the southern Benguela on Malgas Island off South Africa's west coast have experienced large fluctuations in natural prey availability over the past decade, linked to environmental change and localised overfishing. When small pelagic fish are unavailable, breeding gannets increase their consumption of low-quality fishery discards (primarily hake Merluccius spp.). To investigate the limits of foraging flexibility of breeding gannets facing variable prey availability, we monitored foraging behaviour, nest attendance, adult body condition and chick growth between 2002 and 2012, along with diet composition and prey abundance (through annual hydroacoustic assessments) during the birds' breeding season. The combined biomass of sardine Sardinops sagax and anchovy Engraulis encrasicolus within the Malgas gannet colony's foraging range varied tenfold across the study period and was positively correlated with the proportion of these high quality fish in the gannets' diet (17 to 90%). Foraging effort increased and nest attendance decreased with decreasing sardine/anchovy consumption. Adult body condition was negatively impacted by increases in hake in the diet. Chick growth was lowest when low sardine and anchovy composition was coupled with an increase in adult foraging effort, suggesting a limit to behavioural compensation for food shortages. This long-term study demonstrates the consequences of variable prey levels for Cape gannet behaviour and fitness. These results highlight the need for detailed investigations of seabird-fishery interactions, and the necessity to limit fishing within Cape gannet foraging ranges during years of low natural prey abundance. © 2014 Inter-Research.

Louw G.G.,Branch Fisheries Management | Freon P.,French Research Institute for Exploitation of the Sea | Huse G.,Norwegian Institute of Marine Research | Lipinski M.R.,Rhodes University | Coetzee J.C.,Branch Fisheries Management
African Journal of Marine Science | Year: 2014

Patterns in the co-occurrence of small pelagic fish species within single shoals were investigated using data from 6 814 throws of commercial purse-seiners in South Africa. Assuming that the throw composition reflected the true composition of the assemblage, it was shown that: (1) mixed pelagic assemblages were as prevalent as pure shoals; (2) assemblages of anchovy Engraulis encrasicolus and sardine Sardinops sagax exhibited a seasonal distribution pattern; (3) there was a highly skewed species ratio in terms of abundance by mass; and (4) patterns in the size distributions of two-species shoals were complex and dependent on the L∞ and the relative abundance of the species concerned. We hypothesise that the observed patterns reflect the 'net gain of the subordinate', whereby fish occurring in small numbers are less conspicuous and/or less energetically attractive for potential predators if they are smaller than the dominant component of the school. If the subordinate fish grow larger than the dominant fish, this advantage persists. Potential sources of bias are alluded to but are not considered to have had a major impact on the conclusions reached, although they may form the basis for further work. © 2014 Copyright © NISC (Pty) Ltd.

PubMed | University of California at San Diego, Branch Fisheries Management, Nelson Mandela Metropolitan University, Raggy Charters and University of Cape Town
Type: Journal Article | Journal: PloS one | Year: 2015

Studies investigating how mobile marine predators respond to their prey are limited due to the challenging nature of the environment. While marine top predators are increasingly easy to study thanks to developments in bio-logging technology, typically there is scant information on the distribution and abundance of their prey, largely due to the specialised nature of acquiring this information. We explore the potential of using single-beam recreational fish-finders (RFF) to quantify relative forage fish abundance and draw inferences of the prey distribution at a fine spatial scale. We compared fish school characteristics as inferred from the RFF with that of a calibrated scientific split-beam echo-sounder (SES) by simultaneously operating both systems from the same vessel in Algoa Bay, South Africa. Customized open-source software was developed to extract fish school information from the echo returns of the RFF. For schools insonified by both systems, there was close correspondence between estimates of mean school depth (R2 = 0.98) and school area (R2 = 0.70). Estimates of relative school density (mean volume backscattering strength; Sv) measured by the RFF were negatively biased through saturation of this system given its smaller dynamic range. A correction factor applied to the RFF-derived density estimates improved the comparability between the two systems. Relative abundance estimates using all schools from both systems were congruent at scales from 0.5 km to 18 km with a strong positive linear trend in model fit estimates with increasing scale. Although absolute estimates of fish abundance cannot be derived from these systems, they are effective at describing prey school characteristics and have good potential for mapping forage fish distribution and relative abundance. Using such relatively inexpensive systems could greatly enhance our understanding of predator-prey interactions.

Pitcher G.C.,Branch Fisheries Management | Pitcher G.C.,University of Cape Town | Krock B.,Alfred Wegener Institute for Polar and Marine Research | Cembella A.D.,Alfred Wegener Institute for Polar and Marine Research
African Journal of Marine Science | Year: 2011

Diarrhetic shellfish poisoning (DSP) poses a significant threat to the safe consumption of shellfish in the southern Benguela ecosystem. The accumulation of DSP toxins was investigated in two cultivated bivalve species, the Pacific oyster Crassostrea gigas and the mussel Choromytilus meridionalis, suspended from a mooring located off Lambert's Bay on the west coast of South Africa. The dinoflagellate Dinophysis acuminata, a known source of polyether toxins associated with DSP, was common through most of the study period. The toxin composition of the dinoflagellate was dominated by okadaic acid (OA) (91%), with lesser quantities of the dinophysistoxin DTX-1 (6.5%) and pectenotoxin PTX-2 (2.4%), and traces of PTX-2sa and PTX-11. The mean cell toxin quota of D. acuminata was 7.8 pg OA cell -1. The toxin profile in shellfish was characterised by a notably higher relative content of DTX-1. The study showed the average concentration of DSP toxins in the mussels to exceed that in the oysters by approximately 20-fold. The results indicate a need to establish species-specific sampling frequencies in shellfish safety monitoring programmes. © NISC (Pty) Ltd.

de Moor C.L.,University of Cape Town | Johnston S.J.,University of Cape Town | Brandao A.,University of Cape Town | Rademeyer R.A.,University of Cape Town | And 3 more authors.
African Journal of Marine Science | Year: 2015

The waters off South Africa's coastline boast a rich mix of commercially fished species. Quantitative assessments of these marine resources have developed from simple methods first applied in the 1970s, to models that encompass a wide range of methodologies. The more valuable resources have undergone regular assessments in recent decades, with frequencies closely related to the management approach employed for each fishery. Many of these assessments form the operating models used to simulation-test candidate management procedures. This paper provides a comprehensive review of the assessments of 11 of the most important fisheries resources in South Africa. Some assessments use simple biomass dynamics models, whereas others are a hybrid of age- and length-based models, each designed to model the specific characteristics of the resource and fishery concerned. Many of the assessments have been disaggregated by species/stock and/or area as related multispecies/stock/ distribution hypotheses have arisen. This paper explores the similarities and differences in the data available and the methods applied. The review indicates that, whereas the status of three of these resources cannot be estimated reliably at present, the status of six resources is considered to be reasonable to good, whereas that of abalone Haliotis midae and West Coast rock lobster Jasus lalandii remains poor. © 2015 NISC (Pty) Ltd.

Pitcher G.C.,Branch Fisheries Management | Pitcher G.C.,University of Cape Town | Smith M.E.,University of Cape Town | Probyn T.A.,Branch Fisheries Management
African Journal of Marine Science | Year: 2015

Saldanha Bay is a narrow-mouth bay on the west coast of South Africa linked to the southern Benguela upwelling system. Bay productivity was investigated by use of the conventional light-and-dark bottle oxygen method, and, for comparison, through assimilation of the stable isotope tracer 13C. Gross community production GCP and net community production NCP, as determined from the oxygen method, were respectively 2.6 and 2.4 times higher than estimates determined from the stable isotope method. Chlorophyll a (Chl a) concentrations increased with the onset of spring and well-defined subsurface maxima developed in association with increasingly stratified conditions (mean water column Chl a concentrations ranged from 5.4 to 31.5 mg m−3 [mean 15.5 mg m−3; SD 7.6]). A sharp decline in photosynthetic rates P* (GCP normalised to Chl a concentration) with depth was attributed to light limitation, as demonstrated by the high vertical attenuation coefficients for downward irradiance Kd, which varied from 0.29 to 0.70 m−1 (mean 0.48 m−1; SD 0.12). Productivity maxima were consequently near-surface despite the presence of deeper subsurface biomass maxima. The community compensation depth Zcc, where gross community production balances respiratory carbon loss for the entire community, ranged from 2.9 to 9.2 m (mean 5.8 m; SD 2.2), and was typically shallower than the 1% light depth for PAR (photosynthetically available radiation), Z1%PAR, which is traditionally assumed to be the depth of the euphotic zone and which ranged from 6.6 to 15.9 m (mean 9 m; SD 2.6). Autotrophic communities, where organic matter is produced in excess of respiratory demand, were confined on average to the upper 5.8 m of the water column, and often excluded the bulk of the phytoplankton community, where light limitation is considered to lead to heterotrophic community metabolism. Estimates of integrated water column productivity ranged from 0.84 to 8.46 g C m–2 d−1 (mean 3.35 g C m−2 d−1; SD 1.9). © 2015 NISC (Pty) Ltd.

Smith M.E.,University of Cape Town | Pitcher G.C.,Branch Fisheries Management | Pitcher G.C.,University of Cape Town
African Journal of Marine Science | Year: 2015

The efficacy of ocean colour remote sensing in assessing the variability of phytoplankton biomass within Saldanha Bay is examined. Satellite estimates of chlorophyll a (Chl a) were obtained using the maximum peak-height (MPH) algorithm on full-resolution (300 m) data from the Medium Resolution Imaging Spectrometer (MERIS). Subsurface Chl a maxima often occur within Saldanha Bay below the mean detection depth of the satellite (1.5 m) during periods of thermal stratification. Consequently, the MPH product was poorly correlated to in situ data from 4 m depth (r2 and average relative percentage difference [RPD] of 0.094 and 53% respectively); however, the coefficient of determination was much improved if limited to in situ data collected under conditions of mixing (r2 and RPD of 0.869 and 89%, respectively). Composites of monthly MPH Chl a data reveal mean concentrations consistent with in situ seasonal trends of phytoplankton biomass, confirming the capability of the MPH algorithm to qualitatively resolve surface Chl a distribution within the bay. © 2015 NISC (Pty) Ltd.

de Moor C.L.,University of Cape Town | Butterworth D.S.,University of Cape Town | Coetzee J.C.,Branch Fisheries Management
African Journal of Marine Science | Year: 2013

Historically, a time-series of proportions-at-age 1 from annual November hydro-acoustic surveys has been used to inform the assessment of, and in particular the choice of appropriate values for juvenile and adult natural mortality for, the South African anchovy Engraulis encrasicolus. However, information from direct ageing is limited and almost two decades old. A new method is developed to estimate the annual proportions of anchovy at age 1 directly from the population length distributions estimated from samples taken during the November surveys. This method involves modelling the annual length distributions of age 1 and age 2+ anchovy. The analysis provides a new time-series of proportions-at-age 1, together with associated standard errors, for input into assessments of the resource. The results also caution against the danger of scientists reading more information into data than is really there. © 2013 Copyright © NISC (Pty) Ltd.

Everett B.I.,Oceanographic Research Institute | Cliff G.,KwaZulu Natal Sharks Board | Cliff G.,University of KwaZulu - Natal | Dudley S.F.J.,Branch Fisheries Management | And 3 more authors.
African Journal of Marine Science | Year: 2015

Largetooth sawfish Pristis pristis and green sawfish P. zijsron were not uncommon in catches made in KwaZulu‑ Natal (KZN) on the east coast of South Africa in the mid part of the last century but apparently have disappeared from this area. This paper traces the decline in sawfish catches from 1951 and assesses the current population status and local extinction risk, based on historical and current records up to 2012. Records were collected from research surveys, literature, media and museum specimens, and through contacting researchers and conservation managers who have worked in KZN coastal and estuarine areas. A total of 150 green sawfish, 7 largetooth sawfish and 89 unidentified sawfish records were located. Most sawfish (115) were caught during a four‑year (1967–1970) gillnetting survey conducted by the Oceanographic Research Institute in the St Lucia estuarine system while 91 were caught in the bather protection nets installed and maintained along the KZN coast by the KwaZulu‑Natal Sharks Board. Sawfish ranged from 63 to 533 cm total length (TL). Sawfish caught in the estuarine environments (mean TL 162 cm [SD 72], n = 95) were significantly smaller than those caught in the inshore marine environments (mean TL 310 cm [SD 109], n = 83), confirming the importance of estuaries as pupping and nursery areas. The St Lucia estuarine system, given the high abundance of sawfish, was determined to be the most important nursery area in KZN. The last sawfish encountered in KZN, which was not identified to species level, was caught in the bather protection nets in 1999 and released alive. Extinction probability analysis indicates that sawfish no longer occur in KZN waters. Anthropogenic changes to the St Lucia estuarine system, as well as to other KZN estuaries, gillnetting for bather protection, and illegal fish harvesting, coupled with a non‑adaptive life‑history style, may have precipitated the disappearance of sawfish from KZN waters. © 2015 NISC (Pty) Ltd.

Huveneers C.,Flinders University | Ebert D.A.,Moss Landing Marine Laboratories | Ebert D.A.,South African Institute For Aquatic Biodiversity | Dudley S.F.J.,Branch Fisheries Management
African Journal of Marine Science | Year: 2015

Science is continually evolving, with recent developments in some fields, such as conservation biology, leading to shifts in priorities and needs. Recent international conferences focused on chondrichthyan research provide an opportunity to assess how the research environment of chondrichthyan science has evolved through time. We compiled metadata from Sharks Down Under (1991) and the two Sharks International conferences (2010 and 2014), spanning 23 years. Analysis of the data highlighted taxonomic biases towards charismatic species, a declining number of studies in fundamental science such as those related to taxonomy and basic life history, and the emergence of new research fields or tools such as social science and stable isotope analysis. Although there are limitations associated with our study, which are discussed, it lays the foundation for continued assessment of the progression of chondrichthyan research as future chondrichthyan‑focused international conferences are organised. Considering the research biases that our metadata analysis identifies, we suggest that: (i) greater attention should be given to species or species groups that are of particular conservation concern but that may not necessarily be charismatic (e.g. batoids); (ii) increased support should be given to scientists from low‑income countries; (iii) new research areas should continue to be developed and included within broad integrated research programmes; and (iv) concurrent with this, foundational research should not be neglected. © 2015 NISC (Pty) Ltd.

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