Simon Jennings

Rebuilding Global Fisheries

Fighting for Fisheries In the debate concerning the future of the worlds fisheries, some have forecasted complete collapse but others have challenged this view. The protagonists in this debate have now joined forces to present a thorough quantitative review of current trends in world fisheries. Worm et al. (p. 578) evaluate the evidence for a global rebuilding of marine capture fisheries and their supporting ecosystems. Contrasting regions that have been managed for rebuilding with those that have not, reveals trajectories of decline and recovery from individual stocks to species, communities, and large marine ecosystems. The management solutions that have been most successful for rebuilding fisheries and ecosystems, include both large- and small-scale fisheries around the world. Catch restrictions, gear modification, and closed areas are helping to rebuild overexploited marine ecosystems. After a long history of overexploitation, increasing efforts to restore marine ecosystems and rebuild fisheries are under way. Here, we analyze current trends from a fisheries and conservation perspective. In 5 of 10 well-studied ecosystems, the average exploitation rate has recently declined and is now at or below the rate predicted to achieve maximum sustainable yield for seven systems. Yet 63% of assessed fish stocks worldwide still require rebuilding, and even lower exploitation rates are needed to reverse the collapse of vulnerable species. Combined fisheries and conservation objectives can be achieved by merging diverse management actions, including catch restrictions, gear modification, and closed areas, depending on local context. Impacts of international fleets and the lack of alternatives to fishing complicate prospects for rebuilding fisheries in many poorer regions, highlighting the need for a global perspective on rebuilding marine resources.

The effects of fishing on marine ecosystems

ABSTRACT We review the effects of fishing on benthic fauna, habitat, diversity, community structure and trophic interactions in tropical, temperate and polar marine environments and consider whether it is possible to predict or manage fishing-induced changes in marine ecosystems. Such considerations are timely given the disillusionment with some fishery management strategies and that policy makers need a scientific basis for deciding whether they should respond to social, economic and political demands for instituting or preventing ecosystem-based management. Fishing has significant direct and indirect effects on habitat, and on the diversity, structure and productivity of benthic communities. These effects are most readily identified and last longest in those areas that experience infrequent natural disturbance. The initiation of fishing in an unfished system leads to dramatic changes in fish community structure. As fishing intensity increases the additional effects are more difficult to detect. Fishing has accelerated and magnified natural declines in the abundance of many forage fishes and this has lead to reduced reproductive success and abundance in birds and marine mammals. However, such donor-controlled dynamics are less apparent in food webs where fishes are the top predators since their feeding strategies are rather more plastic than those of most birds and mammals. Fishers tend to target species in sequence as a fishery develops and this leads to changes in the composition of the fished communities with time. The dramatic and apparently compensatory shifts in the biomass of different species in many fished ecosystems have often been driven by environmental change rather than the indirect effects of fishing. Indeed, in most pelagic systems, species replacements would have occurred, albeit less rapidly, in the absence of fishing pressure. In those cases when predator or prey species fill a key role, fishing can have dramatic indirect effects on community structure. Thus fishing has shifted some coral reef ecosystems to alternate stable states because there is tight predator–prey coupling between invertebrate feeding fishes and sea urchins. Fishing has reduced, and locally extirpated, populations of predatory fishes. These reductions do not have a consistent effect on the abundance and diversity of their prey: environmental processes control prey populations in some systems, whereas top-down processes are more important in others. By-catch which is discarded during fishing activities may sustain populations of scavenging species, particularly seabirds. We conclude by identifying the circumstances in which new research is needed to guide managers and stress the importance of unfished control sites for studies of fishing effects. We discuss the advantages and disadvantages of closed area management (marine reserves) and the conditions under which such management is likely to provide benefits for the fishery or ecosystem.

Life history correlates of responses to fisheries exploitation

We use an approach based on phylogenetic comparisons to identify life history correlates of abundance trends in 18 intensively exploited fish stocks from the north–east Atlantic. After accounting for differences in fishing mortality, we show that those fishes that have decreased in abundance compared with their nearest relatives mature later, attain a larger maximum size, and exhibit significantly lower potential rates of population increase. Such trends were not evident in a more traditional cross–species analysis. This is the first phylogenetically independent evidence to link life histories with abundance trends, and provides a quantitative basis for assessing vulnerability of fish populations to exploitation. Our approach can be applied to the conservation and management of other exploited taxa.

Current and Future Sustainability of Island Coral Reef Fisheries

Overexploitation is one of the principal threats to coral reef diversity, structure, function, and resilience [1, 2]. Although it is generally held that coral reef fisheries are unsustainable [3-5], little is known of the overall scale of exploitation or which reefs are overfished [6]. Here, on the basis of ecological footprints and a review of exploitation status [7, 8], we report widespread unsustainability of island coral reef fisheries. Over half (55%) of the 49 island countries considered are exploiting their coral reef fisheries in an unsustainable way. We estimate that total landings of coral reef fisheries are currently 64% higher than can be sustained. Consequently, the area of coral reef appropriated by fisheries exceeds the available effective area by approximately 75,000 km(2), or 3.7 times the area of Australias Great Barrier Reef, and an extra 196,000 km(2) of coral reef may be required by 2050 to support the anticipated growth in human populations. The large overall imbalance between current and sustainable catches implies that management methods to reduce social and economic dependence on reef fisheries are essential to prevent the collapse of coral reef ecosystems while sustaining the well-being of burgeoning coastal populations.

The trophic fingerprint of marine fisheries

Biodiversity indicators provide a vital window on the state of the planet, guiding policy development and management. The most widely adopted marine indicator is mean trophic level (MTL) from catches, intended to detect shifts from high-trophic-level predators to low-trophic-level invertebrates and plankton-feeders. This indicator underpins reported trends in human impacts, declining when predators collapse (“fishing down marine food webs”) and when low-trophic-level fisheries expand (“fishing through marine food webs”). The assumption is that catch MTL measures changes in ecosystem MTL and biodiversity. Here we combine model predictions with global assessments of MTL from catches, trawl surveys and fisheries stock assessments and find that catch MTL does not reliably predict changes in marine ecosystems. Instead, catch MTL trends often diverge from ecosystem MTL trends obtained from surveys and assessments. In contrast to previous findings of rapid declines in catch MTL, we observe recent increases in catch, survey and assessment MTL. However, catches from most trophic levels are rising, which can intensify fishery collapses even when MTL trends are stable or increasing. To detect fishing impacts on marine biodiversity, we recommend greater efforts to measure true abundance trends for marine species, especially those most vulnerable to fishing.

Predicting climate-driven regime shifts versus rebound potential in coral reefs

Climate-induced coral bleaching is among the greatest current threats to coral reefs, causing widespread loss of live coral cover. Conditions under which reefs bounce back from bleaching events or shift from coral to algal dominance are unknown, making it difficult to predict and plan for differing reef responses under climate change. Here we document and predict long-term reef responses to a major climate-induced coral bleaching event that caused unprecedented region-wide mortality of Indo-Pacific corals. Following loss of >90% live coral cover, 12 of 21 reefs recovered towards pre-disturbance live coral states, while nine reefs underwent regime shifts to fleshy macroalgae. Functional diversity of associated reef fish communities shifted substantially following bleaching, returning towards pre-disturbance structure on recovering reefs, while becoming progressively altered on regime shifting reefs. We identified threshold values for a range of factors that accurately predicted ecosystem response to the bleaching event. Recovery was favoured when reefs were structurally complex and in deeper water, when density of juvenile corals and herbivorous fishes was relatively high and when nutrient loads were low. Whether reefs were inside no-take marine reserves had no bearing on ecosystem trajectory. Although conditions governing regime shift or recovery dynamics were diverse, pre-disturbance quantification of simple factors such as structural complexity and water depth accurately predicted ecosystem trajectories. These findings foreshadow the likely divergent but predictable outcomes for reef ecosystems in response to climate change, thus guiding improved management and adaptation.

Climate Warming, Marine Protected Areas and the Ocean-Scale Integrity of Coral Reef Ecosystems

Coral reefs have emerged as one of the ecosystems most vulnerable to climate variation and change. While the contribution of a warming climate to the loss of live coral cover has been well documented across large spatial and temporal scales, the associated effects on fish have not. Here, we respond to recent and repeated calls to assess the importance of local management in conserving coral reefs in the context of global climate change. Such information is important, as coral reef fish assemblages are the most species dense vertebrate communities on earth, contributing critical ecosystem functions and providing crucial ecosystem services to human societies in tropical countries. Our assessment of the impacts of the 1998 mass bleaching event on coral cover, reef structural complexity, and reef associated fishes spans 7 countries, 66 sites and 26 degrees of latitude in the Indian Ocean. Using Bayesian meta-analysis we show that changes in the size structure, diversity and trophic composition of the reef fish community have followed coral declines. Although the ocean scale integrity of these coral reef ecosystems has been lost, it is positive to see the effects are spatially variable at multiple scales, with impacts and vulnerability affected by geography but not management regime. Existing no-take marine protected areas still support high biomass of fish, however they had no positive affect on the ecosystem response to large-scale disturbance. This suggests a need for future conservation and management efforts to identify and protect regional refugia, which should be integrated into existing management frameworks and combined with policies to improve system-wide resilience to climate variation and change.

Patterns and prediction of population recovery in marine reserves

Marine reserves (no-take zones) are widely recommended asconservation and fishery management tools. One potential benefitof marine reserves is that they can reduce fishing mortality.This can lead to increases in the abundance of spawners,providing insurance against recruitment failure and maintainingor enhancing yields in fished areas. This paper considers thefactors that influence recovery following marine reserveprotection, describes patterns of recovery in numbers andbiomass, and suggests how recovery rates can be predicted.Population recovery is determined by initial population size, theintrinsic rate of population increase r, and the degree ofcompensation (increases in recruits per spawner as spawnerabundance falls) or depensation (lower than expected recruitmentat low abundance, Allee effect) in the spawner-recruitrelationship. Within a reserve, theoretical recovery rates arefurther modified by metapopulation structure and the success ofindividual recruitment events. Recovery also depends on theextent of reductions in fishing mortality (F) as determined bythe relationship between patterns of movement, migration, anddensity-dependent habitat use (buffer effect) in relation to thesize, shape and location of the reserve. The effects ofreductions in F on population abundance have been calculatedusing a variety of models that incorporate transfer rates betweenthe reserve and fished areas, fishing mortality outside thereserve and life history parameters of the population. Thesemodels give useful indications of increases in production andbiomass (as yield per recruit and spawners per recruitrespectively) due to protection, but do not address recruitment.Many reserves are very small in relation to the geographicalrange of fish or invertebrate populations. In these reserves itmay be impossible to distinguish recovery due to populationgrowth from that due to redistribution. Mean rates of recoverycan be predicted from r, but the methods are data intensive. Thisis ironic when marine reserves are often favoured for managementor conservation in data-poor situations where conventional stockassessment is impossible. In these data-poor situations, it maybe possible to predict recovery rates from very low populationsizes by using maximum body size or age at maturity as simplecorrelates of the intrinsic rate of natural increase.

Impacts of predator depletion by fishing on the biomass and diversity of non-target reef fish communities

Abstract. An understanding of the indirect effects of fishing on predator-prey relationships is required for the development of valid multispecies yield models for reef fisheries and for determining the factors governing fish community structure at larger scales. We used an underwater visual census technique to examine the indirect effects of fishing on the biomass and diversity (species richness) of reef fishes in a series of ten traditional Fijian fishing grounds (qoliqoli) subject to a range of fishing intensities. All members of the families Chaetodontidae (butterflyfishes), Labridae (wrasses), Lutjanidae (snappers), Mullidae (goatfishes), Scaridae (parrotfishes) and the sub-family Epinephelinae (groupers and coral trout) which could be reliably identified were censused. Each species censused was assigned to one of three trophic groups: herbivore, invertebrate feeder or piscivore. The biomass of all piscivorous fishes and of large (>30 cm) piscivorous fishes differed significantly between qoliqoli and was significantly correlated with fishing intensity. However, the biomass of piscivorous fishes was not correlated with the biomass or diversity of their potential prey (which were not targeted by the fishery). This suggested that the indirect effects of fishing did not have an important bearing on fish diversity or biomass and that predation by the target species did not play an important role in structuring these Fijian reef fish communities. The results contrast with those from a number of studies at smaller scales and provided further indications that the structure of reef fish communities is not governed by a single dominant process, but by a range of processes which operate on different scales in different circumstances.

Life–history correlates of maximum population growth rates in marine fishes

Theory predicts that populations of animals with late maturity, low fecundity, large body size and low body growth rates will have low potential rates of population increase at low abundance. If this is true, then these traits may be used to predict the intrinsic rate of increase for species or populations, as well as extinction risks. We used life–history and population data for 63 stocks of commercially exploited fish species from the northeast Atlantic to test relationships between life–history parameters and the rate of population increase at low abundance. We used cross–taxonomic analyses among stocks and among species, and analyses that accounted for phylogenetic relationships. These analyses confirmed that large–bodied, slow–growing stocks and species had significantly lower rates of recruitment and adult production per spawning adult at low abundance. Furthermore, high ages at maturity were significantly correlated with low maximum recruit production. Contrary to expectation, fecundity was significantly negatively related to recruit production, due to its positive relationship with maximum body size. Our results support theoretical predictions, and suggest that a simply measured life–history parameter can provide a useful tool for predicting rates of recovery from low population abundance.