Warren M. Potts

Population Connectivity and Phylogeography of a Coastal Fish, Atractoscion aequidens (Sciaenidae), across the Benguela Current Region: Evidence of an Ancient Vicariant Event

Contemporary patterns of genetic diversity and population connectivity within species can be influenced by both historical and contemporary barriers to gene flow. In the marine environment, present day oceanographic features such as currents, fronts and upwelling systems can influence dispersal of eggs/larvae and/juveniles/adults, shaping population substructuring. The Benguela Current system in the southeastern Atlantic is one of the oldest upwelling systems in the world, and provides a unique opportunity to investigate the relative influence of contemporary and historical mechanisms shaping the evolutionary history of warm-temperate fish species. Using the genetic variation in the mitochondrial DNA Control Region and eight nuclear microsatellite DNA loci, we identified the presence of two highly divergent populations in a vagile and warm-temperate fish species, Atractoscion aequidens, across the Benguela region. The geographical distributions of the two populations, on either side of the perennial upwelling cell, suggest a strong correlation between the oceanographic features of the system and the breakdown of gene flow within this species. Genetic divergence (mtDNA φ ST = 0.902, microsatellite F ST = 0.055: probability of genetic homogeneity for either marker = p<0.001), absence of migrants (less than 1% per generation) between populations and coalescent estimates of time since most recent common ancestor suggest that the establishment of the main oceanographic features of the system (2 million years ago), particularly the strengthening and position of the perennial upwelling cell, is the most likely mechanism behind the observed isolation. Concordance between mitochondrial and nuclear genetic markers indicates that isolation and divergence of the northern and southern Benguela populations of A. aequidens occurred deep in the past and has continued to the present day. These findings suggest that the Benguela Current system may constitute an ancient and impermeable barrier to gene flow for warm-temperate fish species.

Ocean warming, a rapid distributional shift, and the hybridization of a coastal fish species

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.

Evidence of deep genetic divergence between populations of an important recreational fishery species, Lichia amia L. 1758, around southern Africa

Lichia amia is an important coastal recreational fishery species with a cosmopolitan distribution in the eastern Atlantic. In southern Africa, it is distributed from southern Angola to northern KwaZulu-Natal in South Africa. A recent biological survey revealed differences between Angolan and South African individuals suggesting that they may represent separate stocks. As fishery management decisions should be based on accurate knowledge of population structure, an initial survey of the genetic substructuring of L. amia was conducted on samples collected in southern Angola and South Africa. Assessment of genetic diversity and population structure was conducted using a fragment of the mitochondrial Control Region. Obtained genetic diversity levels were within the expected range for marine teleosts (h = 0.867, π = 0.007), and two deeply divergent (southern Angolan and South African) populations were identified (average φ ST = 0.78) across the Benguela Current system. These results suggest that Angolan and South African L. amia should be managed as two independent stocks. This work represents the first assessment of population genetic substructuring across the Benguela Current of a valuable coastal fishery resource, and has application for future fisheries management.

Using baseline biological and ecological information to design a Traffic Light Precautionary Management Framework for leerfish Lichia amia (Linnaeus 1758) in southern Angola

The coastline of southern Angola is sparsely populated and, largely because of a protracted civil war, has very lightly exploited inshore fish stocks. This has provided the almost unprecedented opportunity to determine prefishing-state reference points and to implement management strategies at the early stages of exploitation. The leerfish Lichia amia is distributed from the Mediterranean Sea, along the west coast of Africa to the southern and eastern coasts of South Africa. Despite its importance in recreational catches in South Africa and Angola, there is little information on the biology of this species. The leerfish is one of the three most dominant recreational shore-fishery species in southern Angola, and a biological study on the species was conducted in that region between May 2005 and December 2006. The mean length of captured fish was 767 mm fork length (FL) and 6.8 kg, and the catch per unit effort was 0.13 fish angler−1 h−1 and 0.79 kg angler−1 h−1. The growth of the leerfish population (in mm) was described by: L t = 1 137(1 − e0.22(t + 1.58)), and its total mortality was estimated to be 0.41 (±0.05). Female mortality (0.40 ± 0.06) was lower than that for males (0.44 ± 0.14), and the length and age-at-50% maturity was 623 mm FL and 2.4 years respectively. Female fish with ripe ovaries were found between June and November. The male:female ratio was 1:1.9. Leerfish fed exclusively on fish, mainly sardinella Sardinella aurita (62% frequency of occurrence). Based on this biological information, a theoretical Traffic Light Precautionary Management Framework is constructed for the species. Appropriate management regulations and potential future threats to the species are discussed.

The biology, life history and management needs of a large sciaenid fish, Argyrosomus coronus, in Angola

The West Coast dusky kob Argyrosomus coronus is an understudied yet important fishery species in Angola. During a five-year study (2005–2009), the species was recorded in all fishery sectors, but was most important in the inshore recreational fishery in southern Angola (Cunene Estuary to Namibe). Early juveniles (<300 mm total length, TL) were captured in the offshore artisanal fishery at Praia Pinda, whereas juveniles (300–600 mm TL), subadults (600–870 mm TL) and adults (>870 mm TL) were captured in all fisheries as far north as Namibe, and shoals of large adult fish (>1 000 mm TL) were occasionally captured in the offshore purse-seine commercial fishery between the Cunene Estuary and Lucira. Because some Argyrosomus species are morphologically cryptic, a DNA barcoding method was used to confirm the taxonomic status of the biological samples used in this study. The male:female sex ratio of examined samples was 1:1.4 (n = 225). The length-at-50% maturity was 823 mm and 904 mm TL for males and females respectively. Age-at-50% maturity was 4.4 and 4.3 years for males and females respectively. The periodicity of otolith ring formation was confirmed to be one year using a marginal zone and a chemical marking analysis. Growth (in mm TL) was best described by: Lt = 1 826(1—e−0.12(t + 1.60)). Argyrosomus coronus fed predominantly on fish, mainly Sardinella aurita (62% frequency of occurrence). Early juveniles appeared to frequent the offshore zone (50–100 m depth), moving into the inshore region at approximately 300 mm TL. Juveniles and subadults were resident (57% recaptured at the same site) and were particularly abundant around the mouth of the Cunene Estuary as well as in central and northern Namibia. Adults undertake migrations that correspond with the movement of the Angola–Benguela frontal zone, moving north as far as Gabon in winter and returning to southern Angola in spring, when spawning appeared to take place offshore. There are currently no catch restrictions on A. coronus in Angolan waters. However, declining catches and increasing fishing effort suggest that some management intervention is required, commencing with a proposed closure of the Cunene Estuary mouth region to fishing.

Incipient genetic isolation of a temperate migratory coastal sciaenid fish (Argyrosomus inodorus) within the Benguela Cold Current system

Abstract The Benguela Current is considered to be a major biogeographic barrier for tropical and warm-temperate marine fish, but there is limited knowledge regarding its influence on population sub-structuring of in more cold-tolerant species. Employing genetic variation within the mitochondrial DNA Control Region and six cross-specific nuclear microsatellite markers, a preliminary study was conducted to investigate population sub-structuring in Argyrosomus inodorus, a highly exploited, cold-temperate migratory species, across the Benguela Current region. Results revealed evidence of incipient genetic differentiation (mtDNA ϕST = 0.092; nuclear FST = 0.036 and DST = 0.104, P < 0.05) between the two sampling sites, suggesting the presence of two regional populations. Estimates of contemporary migration rates between populations were low, and similar in range to those reported in tagging surveys. Although preliminary, these results suggest that the oceanographic features of the Benguela Current may have influenced the evolutionary history of A. inodorus, and that the species is likely to be composed of two populations in the Benguela region. As the species is considered overexploited both in Namibia and South Africa, information on the distribution, population dynamics and long-term dispersal patterns across the Benguela Current region would support a comprehensive evaluation of genetic structure, which should be incorporated into fishery management arrangements.

Temporal variability of a temperate fish assemblage in Africa's oldest marine protected area

A standardised research-based, shore-angling survey was conducted in the 47-year-old Tsitsikamma National Park Marine Protected Area (MPA) between 1998 and 2005. The aim of this study was to examine variability in community structure, relative abundance (catch per unit effort, CPUE) and size frequency of the shore-angling fish assemblage in this no-take MPA. A total effort of 8 141 angler hours yielded 8 236 fish of at least 54 species from 19 families. The overall CPUE showed no significant trend over time (r 2 = 0.015, p = 0.1). The interannual variation in CPUE indicated relatively stable populations for some species (e.g. blacktail Diplodus capensis) and patterns of recruitment failure for others (e.g. galjoen Dichistius capensis). However, the lack of a significant trend in taxonomic distinctness (r 2 =0.008, p = 0.8), the stable rank order of species abundance (χ2, r = 361, p < 0.05) and the high coefficient of concordance (W = 0.84) indicated a relatively stable fish assemblage. This was attributed to the extended length of protection afforded by this MPA. One noticeable trend was an increase in the abundance of species with an affinity for warm water and whereas this correlated to an annual increase in water temperature of 0.024 °C between 1998 and 2005, a longer time-series is necessary to identify rise in sea temperature (climate change) as a driver of change in this warm-temperate fish assemblage.

Fish Utilization of Surf-Zones. Are They changing? A Case Study of the Sheltered, Warm-Temperate King's Beach

Surf-zone fish communities and their shifts over time are generally poorly understood. The aim of this study was to compare the current surf-zone fish assemblage at Kings Beach, South Africa, to a similar study conducted three decades ago, before the collapse of many exploited shore fishes in the region. Beach seine nets (mesh sizes of 10 and 50 mm) were used to target juvenile and adult fishes bimonthly from February to August 2011 over the high tide around sunset. A total of 14 species were recorded in both the 30 m and 100 m seine nets. The catch in these seine nets was dominated by Pomadasys olivaceus and Liza richardsonii, and this was significantly different to three decades ago, when P. olivaceus, Sarpa salpa and Diplodus capensis dominated the catch. Important linefish species belonging to the Sparidae and Sciaenidae families were significantly smaller and less abundant in this study. Two sparids, S. salpa and Lithognathus mormyrus, which made a large contribution to the surf-zone catch three decades ago were absent during this study. Reasons for the significant shifts in the surf-zone fish community, including overexploitation of the linefish and potential habitat modification, are discussed.

The life history and fishery potential of Labeo umbratus (Teleostei: Cyprinidae) in five small South African reservoirs

The life histories of five populations of moggel, Labeo umbratus, inhabiting small Eastern Cape reservoirs were compared and the differences related to environmental parameters. A significant positive correlation between fish growth and chlorophyll a was noted in four cases. In the eutrophic Dimbaza Reservoir, slow growth rate was attributed to intraspecific competition. Mortality was lower in populations living amongst fewer predatory species, and also where food was abundant. Length- and age-at-maturity were not affected by environmental factors, but were dependent on growth and mortality. There was a weak relationship between the fishery potential of L. umbratus populations and algal biomass, and thus it was concluded that populations in small, shallow, slightly enriched reservoirs would be those more suitable for exploitation.

Assessment of current data for the octopus resource in Rodrigues, western Indian Ocean

The annual catch of octopus in the Rodrigues Lagoon, located east/north-east of Mauritius, has dropped by almost half since 1994. This has prompted concern over the resource and has initiated an assessment of the current status of the stock. The limited available biological information suggests that the main pulse of recruitment into the lagoon occurs in the first few months of the year, but that exact timing varies between years and additional staggered recruitment may occur throughout the year. Size frequency information suggests that the brooding period may be variable and extended, with a main peak between October and January. Catch-and-effort data were also limited, but a decreasing trend in catch per unit effort in two of the datasets examined allowed some preliminary within-season assessment of population size (using a depletion model) at selected landing sites. The use of a migration bias correction formula suggested a net emigration from the lagoon, which might be indicative of adult movement into deeper waters. Depletion models seem to be useful as a management tool and should be tested further against more substantial monthly catch-and-effort data. In the interim, a precautionary approach of instituting a national seasonal closure for octopus fishing following periods of peak recruitment may have both economic and conservation benefits.