Review of size- and age-dependence in batch spawning: implications for stock assessment of fish species exhibiting indeterminate fecundity

Most assessments of fish stocks use some measure of the reproductive potential of a population, such as spawning biomass. However, the correlation between spawning biomass and reproductive potential is not always strong, and it likely is weakest in the tropics and subtropics, where species tend to exhibit indeterminate fecundity and release eggs in batches over a protracted spawning season. In such cases, computing annual reproductive output requires estimates of batch fecundity and the annual number of batches—the latter subject to spawning frequency and duration of spawning season. Batch fecundity is commonly measured by age (or size), but these other variables are not. Without the relevant data, the annual number of batches is assumed to be invariant across age. We reviewed the literature and found that this default assumption lacks empirical support because both spawning duration and spawning frequency generally increase with age or size. We demonstrate effects of this assumption on measures of reproductive value and spawning potential ratio, a metric commonly used to gauge stock status. Model applications showed substantial sensitivity to age dependence in the annual number of batches. If the annual number of batches increases with age but is incorrectly assumed to be constant, stock assessment models would tend to overestimate the biological reference points used for setting harvest rates. This study underscores the need to better understand the age- or size-dependent contrast in the annual number of batches, and we conclude that, for species without evidence to support invariance, the default assumption should be replaced with one that accounts for age- or size-dependence.

Population genetics of Cobia (Rachycentron canadum): implications for fishery management along the coast of the southeastern United States

Cobia (Rachycentron canadum) is a pelagic, migratory species with a transoceanic distribution in tropical and subtropical waters. Recreational fishing pressure on Cobia in the United States has increased substantially during the last decade, especially in areas of its annual inshore aggregations, making this species potentially susceptible to overfishing. Although Cobia along the Atlantic and Gulf coasts of the southeastern United States are currently managed as a single fishery, the genetic composition of Cobias in these areas is unclear. On the basis of a robust microsatellite data set from collections along the U.S. Atlantic coast (2008–09), offshore groups were genetically homogenous. However, the 2 sampled inshore aggregations (South Carolina and Virginia) were genetically distinct from each other, as well as from the offshore group. The recapture of stocked fish within their release estuary 2 years after release indicates that some degree of estuarine fidelity occurs within these inshore aggregations and supports the detection of their unique genetic structure at the population level. These results complement the observed high site fidelity of Cobias in South Carolina and support a recent study that confirms that Cobia spawn in the inshore aggregations. Our increased understanding of Cobia life history will be beneficial for determining the appropriate scale of fishery management for Cobia.

Increased toxicity of Karenia brevis during phosphate limited growth: ecological and evolutionary implications

Karenia brevis is the dominant toxic red tide algal species in the Gulf of Mexico. It produces potent neurotoxins (brevetoxins [PbTxs]), which negatively impact human and animal health, local economies, and ecosystem function. Field measurements have shown that cellular brevetoxin contents vary from 1–68 pg/cell but the source of this variability is uncertain. Increases in cellular toxicity caused by nutrient-limitation and inter-strain differences have been observed in many algal species. This study examined the effect of P-limitation of growth rate on cellular toxin concentrations in five Karenia brevis strains from different geographic locations. Phosphorous was selected because of evidence for regional P-limitation of algal growth in the Gulf of Mexico. Depending on the isolate, P-limited cells had 2.3- to 7.3-fold higher PbTx per cell than P-replete cells. The percent of cellular carbon associated with brevetoxins (%C-PbTx) was ~ 0.7 to 2.1% in P-replete cells, but increased to 1.6–5% under P-limitation. Because PbTxs are potent anti-grazing compounds, this increased investment in PbTxs should enhance cellular survival during periods of nutrient-limited growth. The %C-PbTx was inversely related to the specific growth rate in both the nutrient-replete and P-limited cultures of all strains. This inverse relationship is consistent with an evolutionary tradeoff between carbon investment in PbTxs and other grazing defenses, and C investment in growth and reproduction. In aquatic environments where nutrient supply and grazing pressure often vary on different temporal and spatial scales, this tradeoff would be selectively advantageous as it would result in increased net population growth rates. The variation in PbTx/cell values observed in this study can account for the range of values observed in the field, including the highest values, which are not observed under N-limitation. These results suggest P-limitation is an important factor regulating cellular toxicity and adverse impacts during at least some K. brevis blooms.

Impact of the invasive colonial tunicate Didemnum vexillum on the recruitment of the bay scallop (Argopecten irradians irradians) and implications for recruitment of the sea scallop (Placopecten magellanicus) on Georges Bank

The invasive colonial tunicate Didemnum vexillum has become widespread in New England waters, colonizing large areas of shell-gravel bottom on Georges Bank including commercial sea scallop (Placopecten magellanicus) grounds. Didemnum vexillum colonies are also fouling coastal shellfish aquaculture gear which increases maintenance costs and may affect shellfish growth rates. We hypothesized that D. vexillum will continue to spread and may affect shellfish larval settlement and survival. We conducted a laboratory experiment to assess interactions between larval bay scallops (Argopectin irradians irradians) and D. vexillum. We found that larval bay scallops avoid settling on D. vexillum colonies, possibly deterred by the low pH of the tunicate’s surface tissue. The results of this study suggest that widespread colonization of substrata by D. vexillum could affect scallop recruitment by reducing the area of quality habitats available for settlement. We propose that the bay scallop can serve as a surrogate for the sea scallop in estimating the negative impact D. vexillum could have on the recruitment of sea scallops on Georges Bank.

The past, present and projected scenarios in the Lake AIbert and Albert Nile fisheries: implications for sustainable management

Lake Albert is one of the largest lakes in Uganda that still supports a multi-species fishery which as a result of variable adult sizes of the species, causes management challenges especially in relation to gear mesh size enforcement. Prior to the 1980s, commercial species were 17 largesized fishes especially Citharinus citharinus, Distichodus niloticus and Lates spp. that were confmed to inshore habitats of the lake and were thus rapidly over fished. Frame and catch assessment surveys conducted in this study revealed a >80% dominance of small size fish species (Neobola bredoi and Brycinus nurse) and a 40 -60% decrease in the contribution of the large commercial species. Sustainability of small size fish species is uncertain due to seasonal fluctuations and low beach value. At about 150,000 tons of fish recorded from Lake Albert and Albert Nile, the beach value was estimated at 55.3 million USD. Despite the noted decline in catches of the large sized fishes their contribution was more than 50% of total beach value. Therefore, management measures should couple value addition for the small sized species and maintain effort regulation targeting recovery of the large previously important commercial species

Implications of changes in trophic diversity and food webs on fisheries and the environment

Most of the earth's ecosystems are experiencing slight to catastrophic losses of biodiversity, caused by habitat destruction, alien species introduction, climate change and pollution (Wilcove et al., 1998). These human effects have led to the extinction of native fish species, the collapse of their populations and the loss of ecological integrity and ecosystem functioning (Ogutu-Ohwayo & Hecky, 1991; Witte et al. , 1992a; Mills et al., 1994; Vitousek et al., 1996). Food webs are macro-descriptors of community feeding interactions that can be used to map the flow of materials and nutrients in ecosystems (Jepsen & Winemiller, 2002). Comparative food web studies have been used to address theoretical questions such as 'does greater trophic connectivity increase stability?' (Cohen et al., 1990), and 'does the number of trophic levels increase with productivity?' (Briand & Cohen, 1987). Answers to such questions have obvious applications for natural resources management. From a multi-species fisheries standpoint, there is a need to understand consumer-resource dynamics within complex trophic networks.

Implications of the industrial fish processing growth for the commercial fishery and fishers in Uganda

The study was undertaken to generate socio-economic information on fish market systems and performance of the industrial processing industry, which will guide the processes leading to modernization of the fisheries sector and, sustainability of Lake Victoria fisheries. The main objective of this study was to evaluate the socio-economic implications of the fish marketing systems with particular emphasis on fish export market in Uganda. The study thus, analysed the socio-economic characteristics of fishers and examinined fish marketing systems and the impacts on the fishing activities, food security, employment opportunities and incomes of fisher-folk communities.

Futures of inland aquatic agricultural systems and implications for fish agri-food systems in southern Africa

The CGIAR Research Program on Aquatic Agricultural Systems (AAS) is collaborating with partners to develop and implement a foresight-based engagement with diverse stakeholders linked to aquatic agricultural systems. The program’s aim is to understand the implications of current drivers of change for fish agri-food systems, and consequently food and nutrition security, in Africa, Asia and the Pacific. Partners include the Global Forum on Agricultural Research (GFAR), the Forum for Agricultural Research in Africa (FARA) and the African Union’s New Partnership for Africa’s Development (AU-NEPAD). A key part of the program was a participatory scenario-building workshop held in July 2015 under the theme of "futures of aquatic agricultural systems and implications for fish agri-food systems in southern Africa." The objectives for the workshop were (i) to engage local stakeholders in exploring plausible futures of aquatic agricultural systems, and (ii) to broker and catalyze collaborative plans of action based on the foresight analysis. This report presents technical findings from the workshop. The CGIAR Research Program on Aquatic Agricultural Systems (AAS) is collaborating with partners to develop and implement a foresight-based engagement with diverse stakeholders linked to aquatic agricultural systems. The program’s aim is to understand the implications of current drivers of change for fish agri-food systems, and consequently food and nutrition security, in Africa, Asia and the Pacific. Partners include the Global Forum on Agricultural Research (GFAR), the Forum for Agricultural Research in Africa (FARA) and the African Union’s New Partnership for Africa’s Development (AU-NEPAD). A key part of the program was a participatory scenario-building workshop held in July 2015 under the theme of "futures of aquatic agricultural systems and implications for fish agri-food systems in southern Africa." The objectives for the workshop were (i) to engage local stakeholders in exploring plausible futures of aquatic agricultural systems, and (ii) to broker and catalyze collaborative plans of action based on the foresight analysis. This report presents technical findings from the workshop.

The past, present and projected scenarios in the Lake Albert and Albert Nile fisheries: implications for sustainable management

Lake Albert is one of the largest lakes in Uganda that still supports a multi-species fishery which as a result of variable adult sizes of the species, causes management challenges especially in relation to gear mesh size enforcement. Prior to the 1980s, commercial species were 17 large sized fishes especially Citharinus citharinus, Distichodus niloticus and Lates spp. that were confimed to inshore habitats of the lake and were thus rapidly over fished. Frame and catch assessment surveys conducted in this study revealed a >80% dominance of small size fish species (Neobola bredoi and Brycinus nurse) and a 40 -60% decrease in the contribution of the large commercial species. Sustainability of small size fish species is uncertain due to seasonal fluctuations and low beach value. At about 150,000 tons of fish recorded from Lake Albert and Albert Nile, the beach value was estimated at 55.3 million USD. Despite the noted decline in catches of the large sized fishes their contribution was more than 50% of total beach value. Therefore, management measures should couple value addition for the small sized species and maintain effort regulation targeting recovery of the large previously important commercial species.

Fishing effort and fish yield over a 15 year period on Lake Victoria, Uganda: management implications

The National Fisheries Resources Research Institute (NaFIRRI), the Directorate of Fisheries Resources (DiFR), the Local Government fisheries staff and those from the Beach Management Units (BMUs) of the riparian districts to Lake Victoria regularly and jointly conduct Frame and Catch Assessment Surveys. The information obtained is used to guide fisheries management and development. We reveal the trends in the commercial fish catch landings and fishing effort on the Uganda side of Lake Victoria, over a 15 year period (2000-2015) and provide the underlying factors to the observed changes. The contribution of the high value large size species (Nile perch and Tilapia) to the commercial catch of Lake Victoria has significantly reduced while that of the low value small size species, Mukene has increased over a ten year (2005-2015)period. The information is intended to update and sensitize the key stakeholders on the status of the Lake Victoria fisheries. In addition, the information provided is expected to guide policy formulation and management planning by the fisheries managers at all levels including the BMUs and Landing Site Management Committees (LSMCs), the Local government fisheries staff and the Directorate of Fisheries Resources. The information is anticipated to create awareness among the lakeside fisher communities to reverse the current trend in fish declines.