Sven E. Kerwath

Heavy metals in marine fish meat and consumer health: a review

The numerous health benefits provided by fish consumption may be compromised by the presence of toxic metals and metalloids such as lead, cadmium, arsenic and mercury, which can have harmful effects on the human body if consumed in toxic quantities. The monitoring of metal concentrations in fish meat is therefore important to ensure compliance with food safety regulations and consequent consumer protection. The toxicity of these metals may be dependent on their chemical forms, which requires metal speciation processes for direct measurement of toxic metal species or the identification of prediction models in order to determine toxic metal forms from measured total metal concentrations. This review addresses various shortcomings in current knowledge and research on the accumulation of metal contaminants in commercially consumed marine fish globally and particularly in South Africa, affecting both the fishing industry as well as fish consumers.

Crossing Invisible Boundaries: the Effectiveness of the Langebaan Lagoon Marine Protected Area as a Harvest Refuge for a Migratory Fish Species in South Africa

The application of no-take areas in fisheries remains controversial. Critics argue that many targeted species are too mobile to benefit from area protection and that no-take areas are only appropriate for resident species. The degree of protection does not depend on the size of the no-take area but rather on the time fish reside inside its boundaries during key life-history events (i.e., spawning) and during periods of peak fishing activity. We evaluated the potential of a small no-take marine protected area (MPA) inside a coastal embayment as a harvest refuge for a mobile, possibly migratory, long-lived fish species. We used acoustic telemetry to track movements of 30 transmitter-tagged white stumpnose (Rhabdosargus globiceps) across and on both sides of the boundary of a small (34 km(2)) no-take area over a full year. Being landlocked on 3 sides, the location of the MPA inside the lagoon made it practical to detect all boundary crossings and to calculate the time individual fish used the MPA. We detected frequent movements across the boundary, with strong seasonal and individual variations. There were significant differences in MPA use patterns between fish from different release areas. The time spent in the MPA by individual fish during summer (mean 50%; max 98%) was out of proportion with the size of that area (4% of total habitat). Summer coincided with peak recreational fishing activity and with the spawning season of this species. The small MPA provided a refuge for a part of the spawning stock of white stumpnose. Our findings suggest that if strategically placed, a small no-take area can be effective in protecting mobile species and that models of spillover from no-take areas should account for seasonal and individual variation in area use and the spatiotemporal distribution of fish and fishers.

Mercury accumulation in Yellowfin tuna (Thunnus albacares) with regards to muscle type, muscle position and fish size.

The concentrations and relationships between individual mercury species and total mercury were investigated in different muscle parts and sizes of Yellowfin tuna (Thunnus albacares). Fourteen Yellowfin tuna caught in the South Atlantic off the coast of South Africa had an average total Hg (tHg) concentration of 0.77 mg/kg wet weight. No differences were detected (p > 0.05) in tHg, MethylHg (MeHg) or inorganic Hg (iHg) accumulation among the four white muscle portions across the carcass, but both tHg and iHg were found in higher concentrations (p < 0.001) in dark muscle than white muscle. Positive linear correlations with fish weight were found for both tHg (r = 0.79, p < 0.001) and MeHg (r = 0.75, p < 0.001) concentrations. A prediction model was formulated to calculate toxic MeHg concentrations from measured tHg concentrations and fish weight (cMeHg = 0.073 + 1.365 · tHg-0.008 · w). As sampling sites and subsampling methods could affect toxicity measurements, we provide recommendations for sampling guidelines.

Long-term effects of two sizes of surgically implanted acoustic transmitters on a predatory marine fish (Pomatomus saltatrix)

Fish telemetry is increasingly used to study fish behaviour in marine systems and it is crucial that the tagging does not affect fish behaviour and welfare negatively. Hence, the long-term effects of surgically implanted acoustic telemetry transmitters on survival, tag retention, healing and growth on Pomatomus saltatrix (known as tailor in Australia, elf in South Africa and bluefish in North America) were studied over 144 days. P. saltatrix are well suited for transmitter implants because no tagging-related short- or long-term mortality or transmitter expulsion was recorded. Small transmitters (9 × 28 mm, 3.3 g in water) did not affect the specific growth rate of tagged fish compared with an untagged control group. In contrast, the fish tagged with large transmitters (13 × 50 mm, 6.9 g in water) had a reduced specific growth rate. The small, but not the large transmitter, seemed suitable for tagging P. saltatrix of the body sizes represented in this study (215–621 g). The specific growth rate was negatively affected by increasing the transmitter-mass-in-water to body-mass ratio. It is recommended that this ratio should not exceed 1.4%.

Effects of fishing on a temperate reef community in South Africa 1: ichthyofauna

Exploitation of temperate reef fish not only affects the target species but potentially changes the composition of reef fish assemblages. This study investigated the effect of fishing on the ichthyofaunal community at protected and exploited sites around the Goukamma Marine Protected Area (MPA) on the South African temperate south coast. Abundance and size frequency data from 273 standardised angling and 44 underwater visual census sites were analysed with generalised linear modelling and multivariate techniques to separate the effects of exploitation, habitat, time of day and season. Ichthyofaunal communities differed significantly between protected and exploited sites, which is likely a result of differential fishing pressure. Abundance and size of roman Chrysoblephus laticeps, the dominant species of the local linefishery, were significantly lower at exploited sites. Non-target species with considerable dietary overlap with roman were significantly less abundant inside the MPA, probably due to interspecific competition, whereas species whose diets differed to that of roman were more abundant at protected sites. The results show that fishing can have an influence on the composition of target and non-target species and that these effects need to be considered in fisheries management and conservation planning.

Quantifying the degree of protection afforded by a no-take marine reserve on an exploited shark

Sharks have been shown to benefit from the protection of marine protected areas (MPAs). There is, however, little information on the degree of protection by MPAs to shark populations. The movements of individual smoothhound sharks Mustelus mustelus in, and adjacent to, a small (34 km2) no-take MPA (Langebaan Lagoon Marine Protected Area; LMPA) situated on the west coast of South Africa were investigated over two years using acoustic telemetry. Sharks spent the majority of the time (in hours, average 79%) inside the reserve, and some sharks (n = 2 of 15 recorded during a full year) did not leave the reserve during the study period. Time spent inside the LMPA and the number of crossings of its boundary were strongly influenced by season. Sharks concentrated inside the LMPA during summer, whereas they were widely distributed throughout the study area during winter. Six sharks left the Saldanha Bay embayment during spring and winter for durations ranging from two to 156 days (median = 111 days). All returned to the bay within the study period. Individuals recorded over two years showed consistency in behavioural patterns and protection by the LMPA between years, and spent an average of 74% and 80% of the time inside the LMPA during the two study years respectively. The extended residency of smoothhound sharks within the LMPA suggests that no-take area protection may be a viable management option.

The effect of marine protected areas on an exploited population of sex-changing temperate reef fish: an individual-based model

The effect of two marine protected areas (MPAs) on roman Chrysoblephus laticeps (Sparidae), an exploited reef-fish species inhabiting the South African temperate south coast, was simulated with a spatially explicit, individual-based model based on geographically correct habitat distribution. The model domains were small compared with the dispersal range of pre-recruits, and recruitment was held constant. Adult fish movement rules were based on the results of high-resolution mark-and-recapture and telemetry studies. Fish densities, age-structure and life-history parameters were derived from comprehensive underwater surveys and biological analyses. The model included the effect of fishing on size-at-sex change. The results indicate a recovery of fish abundance-per-recruit, size frequency and sex ratio to, or close to, pre-exploitation levels within the protected areas 10 years after the implementation of both MPAs. Results suggest that, for resident species such as roman, even small MPAs (6 km2) can offer protection. The small exchange of post-recruit fish into the adjacent areas results in negligible improvement (1%) of catches. The MPAs support more and larger spawning fish, which may improve recruitment into adjacent fished areas

Comparison of the effects of different linefishing methods on catch composition and capture mortality of South African temperate reef fish

The effects of hook size, bait type, water depth and fishing effort on the selectivity and capture mortality of reef fish were investigated in a standardised fishing experiment on the temperate south coast of South Africa. A total of 2 406 fish was caught at 273 stations, distributed according to a random-stratified design. Generalised linear models separated the influences of environmental factors such as depth, temperature and season from the effects of fishing methods on catch rates and capture mortality. Dominant reef predators and larger individuals within species were more frequently caught on large hooks and sardine Sardinops sagax bait. The main cause for capture mortality was gut-hooking, aggravated by increased capture depth. Large roman Chrysoblephus laticeps were more frequently gut-hooked than small individuals. More roman were caught early in the fishing session and the average size of captured roman increased slightly during fishing sessions. The selection of legal-sized target fish can be increased by about 23% and immediate capture mortality of undersized fish can be reduced by 50% by using large hooks and sardine bait and by fishing for less than one angler-hour per station in water shallower than 20m. The results are useful for planning fisheries independent catch per unit effort surveys and for developing rules for catch-and-release competitions.

Heavy metal accumulation and toxicity in smoothhound (Mustelus mustelus) shark from Langebaan Lagoon, South Africa.

Together with several health benefits, fish meat could introduce toxins to consumers in the form of heavy metal contaminants. High levels of mercury (Hg), especially, are frequently detected in certain predatory fish species. Mustelus mustelus fillets were analysed for 16 metals and three individual Hg species (inorganic Hg, ehtylmercury, methylmercury) with inductively coupled plasma mass spectrometry (ICP-MS) and HPLC-ICP-MS respectively. Eleven of the 30 sharks had total Hg levels above the maximum allowable limit with toxic methylmercury found as the dominant mercury species with a strong correlation (r = 0.97; p < 0.001) to total mercury concentrations. Limited correlations between metals and shark size parameters were observed; therefore metal accumulation in M. mustelus is mostly independent of size/age. Average values for arsenic (28.31 ± 18.79 mg/kg) exceed regulatory maximum limits and Hg (0.96 ± 0.69 mg/kg) is close to the maximum limit with all other metals well below maximum limits.

First documented southern transatlantic migration of a blue shark Prionace glauca tagged off South Africa

The first documented recapture of a South African-tagged juvenile blue shark Prionace glauca off Uruguay lends weight to the hypothesis of a single blue shark population in the South Atlantic. The presence of neonate blue sharks with umbilical scars and females with post-parturition scars, as well as the high frequency of small juveniles in research longline catches, confirm the existence of a parturition and nursery area off South Africa. The final positions of three tagged sharks suggest that large-scale movement patterns in the South Atlantic are a mirror image of movements in the North Atlantic, with sharks using the north-westerly Benguela Drift to migrate into the tropics and ultimately across into South American waters. The confirmed existence of a parturition and nursery area off the south coast of South Africa and the movement of sharks into both adjacent ocean basins suggest that the southern African blue sharks are part of a single stock that straddles the South Atlantic and Indian oceans, and possibly the entire Southern Hemisphere.