On the Optimal Size of Marine Reserves

The excessive and unsustainable exploitation of our marine resources has led to the promotion of marine reserves as a fisheries management tool. Marine reserves, areas in which fishing is restricted or prohibited, can offer opportunities for the recovery of exploited stock and fishery enhancement. This study examines the impact of the creation of marine protected areas, from both economic and biological perspectives. The consequences of reserve establishment on the long-run equilibrium fish biomass and fishery catch levels are evaluated. We include reserve size as control variable to maximize catch at equilibrium. A continuous time model is used to simulate the effects of reserve size on fishing catch. Fish movements between the sites is assumed to take place at a faster time scale than the variation of the stock and the change of the fleet size. We take advantage of these two time scales to derive a reduced model governing the dynamics of the total fish stock and the fishing effort. Simulation results suggest that the establishment of a protected marine reserve will always lead to an increase in total fish biomass, an optimal size of a marine reserve can achieve to maximize the catch at equilibrium.     

The state of exploitation of the commercial demersal fishery of Cameroon

The state of exploitation of the demersal fisheries resources of Cameroon has been assessed using the classic Schaefer's (1954) model and the Gulland'ss (1961) moving average. The euilibrium yield found with the Schaefer method is statistically different (95 % probability) and higher than the Gulland approach. Because equilibrium models consistently over-estimate MSY and its related optimum effort, management option should target the 95 % value of the estimated parameters. The resources are being over-fished; as an immediate alternative to the urgent concern, catch and/or effort quotas could be allocated to the various fishing companies, with the total allocated catch and/or effort (for all fishing companies) 5% less than the estimated parameters. Enforcement control of that policy would be simplified as fishing activities are localised in the two main estuaries of the “Cameroon River” and Riodel-Rey;results should be complemented by economic studies of the fishery, as these economic factors would explain or better predict the behaviour of the fishing industry.     

A plan for fisheries management in the lakes drained by the Oulujoki river

Freshwater fishery management is treated as a dynamic system comprising environment protection and improvement, fishing, fishery resources allocation, fish stocking programs and marketing policy. The aims of the plan are to increase the economic value of the catch and to protect the professional fishery. Fisheries statistics, catch per unit effort data and other material were collected during 1972–1976. The total allowable catch (TAC) for the most important fish species was estimated with MSY and population analysis models. The results of fish stockings were studied by tagging and population analysis calculations. Fishing of vendace and non-valuable species (perch, roach, smelt) can be increased, but the fishing pressure on other species should not be raised above the present level. Restrictions on whitefish fishing are needed in some areas. A balanced multispecies fishery is desirable, and suggestions are given for the composition of the fishing gear. Fish stocking can make possible a larger and more valuable catch, but at present its profitability is rather low. The stocking results are strongly affected by the fishery and the gear composition.     

The consequences of balanced harvesting of fish communities

Balanced harvesting, where species or individuals are exploited in accordance with their productivity, has been proposed as a way to minimize the effects of fishing on marine fish communities and ecosystems. This calls for a thorough examination of the consequences balanced harvesting has on fish community structure and yield. We use a size- and trait-based model that resolves individual interactions through competition and predation to compare balanced harvesting with traditional selective harvesting, which protects juvenile fish from fishing. Four different exploitation patterns, generated by combining selective or unselective harvesting with balanced or unbalanced fishing, are compared. We find that unselective balanced fishing, where individuals are exploited in proportion to their productivity, produces a slightly larger total maximum sustainable yield than the other exploitation patterns and, for a given yield, the least change in the relative biomass composition of the fish community. Because fishing reduces competition, predation and cannibalism within the community, the total maximum sustainable yield is achieved at high exploitation rates. The yield from unselective balanced fishing is dominated by small individuals, whereas selective fishing produces a much higher proportion of large individuals in the yield. Although unselective balanced fishing is predicted to produce the highest total maximum sustainable yield and the lowest impact on trophic structure, it is effectively a fishery predominantly targeting small forage fish.     

Sustainable exploitation of temperate fish stocks

The theory of maximum sustainable yield (MSY) underpins many fishery management regimes and is applied principally as a single species concept. Using a simple dynamic biomass production model we show that MSY can be identified from a long time series of multi-stock data at a regional scale in the presence of species interactions and environmental change. It suggests that MSY is robust and calculable in a multispecies environment, offering a realistic reference point for fishery management. Furthermore, the demonstration of the existence of MSY shows that it is more than a purely theoretical concept. There has been an improvement in the status of stocks in the Northeast Atlantic, but our analysis suggests further reductions in fishing effort would improve long-term yields.     

The stabilizability of a controlled system describing the dynamics of a fishery

This work presents two stock-effort dynamical models describing the evolution of a fish population growing and moving between two fishing zones, on which it is harvested by a fishing fleet, distributed on the two zones. The first model corresponds to the case of constant displacement rates of the fishing effort, and the second one to fish stock-dependent displacement rates. In equations of the fishing efforts, a control function is introduced as the proportion of the revenue to be invested, for each fleet. The stabilizability analysis of the aggregated model, in the neighborhood of the equilibrium point, enables the determination of a Lyapunov function, which ensures the existence of a stabilizing discontinuous feedback for this model. This enables us to control the system and to lead, in an uniform way, any solution of this system towards this desired equilibrium point.     

Fishing at Rio Grande (Brazil): Ecological niche and competition

This study is an ecological analysis of the use of resources by two potentially competitive groups, the commercial and recreational fishermen of Rio Grande, located on the border of the States of SÃo Paulo and Minas Gerais. Tourism at Rio Grande was intensified after the construction of the hydroeletric plant of Marimbondo in 1974, and local fishermen have complained about its interference. Values of niche dimensions, such as fishing periods, grounds, and prey caught were obtained through data gathered at landing points. Local and recreational fishermen usually fish in different parts of the river and catch different fish, because they use different gear. However, in transition months between the wet and dry seasons, when fish are less abundant, there is conflict or competition related to fishing grounds, reinforced by the territorial fishing rights of local longline fishermen in this period. We show how ecological concepts, such as niche and competition, may help to understand strategies of exploitation by human groups, fundamental information for the management of tropical areas.     

Fisheries for small pelagic species: an empirical approach to management targets

Summary Long-term management targets based on MSY, F_max or F_0.1 are inappropriate for small pelagic fish because of the possibility of stock collapse owing to a stock-recruit relationship at low biomasses. Better reference points such as F_med and F_high that take account of stock and recruit data cannot be used in developing fishery situations because they are too demanding of data. A simple model was fitted to medium-term (about 10 year) periods in exploited small pelagic fisheries, relating change in stock biomass to exploitation rate. Data from 28 stocks and 11 species were used. The fitted model was used to estimate likelihood of stock decrease at different exploitation rates. The pelagic stocks included in the model appeared to be in equilibrium for an exploitation rate F/Z=0.4, which may be used as a guideline for the appropriate exploitation of pelagic stocks.     

Competition and coexistence in spatially subdivided habitats

While non-spatial models predict that like species cannot stably coexist, empirical studies suggest that similar species have similar distributions due to shared habitat requirements. A model is developed to discuss competition and coexistence in subdivided but locally stable habitats. The model predicts that in some cases it is possible for one species to exclude the other species from a geographic region, while in other cases two competing species can stably coexist. The equilibrium level and the fraction of doubly occupied patches, if there is coexistence, are determined by the strength of competition on colonization and exclusion in such a system. Also, it is possible for two ecologically identical species to stably coexist, and two asymmetrically competing species can coexist when there is a trade-off between local competition ability and invasion ability. When rescue effects are considered, the stable region at internal equilibrium point would be reduced, but the fraction of doubly occupied patches would be enlarged.     

Catch and discards from experimental trawl and longline fishing in the deep water of the Rockall Trough

Comparative bottom trawl and longline surveys were carried out on two chartered commercial fishing vessels in the deep waters (350-1300 m) of the Rockall Trough, an area subjected to heavy commercial exploitation. The species composition, catch rates and length distributions from each survey were very different and reflected the fundamental difference in the two types of fishing operations. Bottom-trawled catches produced greater species diversity and higher discard rates. Longline catches produced larger specimens of teleost fish and were dominated by squalid shark. Trawl discards, expressed as kgs of discards per tonne of roundnose grenadier Coryphaenoides rupestris landed, were calculated for a broad range of the most abundant species taken in the catch. First estimates of total international discarding from deep-water trawling operations in the Rockall Trough area (7530 tonnes; 26.5 million individuals) were made by raising the discard rates using international grenadier landings for 1995. The outlook for the continued exploitation of the deep-water fish resource in the Rockall Trough and possible management options are discussed.     

Sustainability in single-species population models

In this paper, we review the concept of sustainability with regard to a single-species, age-structured fish population with density dependence at some stage of its life history. We trace the development of the view of sustainability through four periods.The classical view of sustainability, prevalent in the 1970s and earlier, developed from deterministic production models, in which equilibrium abundance or biomass is derived as a function of fishing mortality. When there is no fishing mortality, the population equilibrates about its carrying capacity. We show that carrying capacity is the result of reproductive and mortality processes and is not a fixed constant unless these processes are constant. There is usually a fishing mortality, F(MSY), which results in MSY, and a higher value, F(ext), for which the population is eventually driven to extinction. For each F between 0 and F(ext), there is a corresponding sustainable population. From this viewpoint, the primary tool for achieving sustainability is the control of fishing mortality.The neoclassical view of sustainability, developed in the 1980s, involved population models with depensation and stochasticity. This view point is in accord with the perception that a population at a low level is susceptible to collapse or to a lack of rebuilding regardless of fishing. Sustainability occurs in a more restricted range from that in the classical view and includes an abundance threshold. A variety of studies has suggested that fishing mortality should not let a population drop below a threshold at 10-20% of carrying capacity.The modern view of sustainability in the 1990s moves further in the direction of precaution. The fishing mortality limit is the former target of F(MSY) (or some proxy), and the target fishing mortality is set lower. This viewpoint further reduces the range of permissible fishing mortalities and resultant desired population sizes. The objective has shifted from optimizing long-term catch to preserving spawning biomass and egg production for the future. The use of discount rates in objective functions involving catch is not a suitable alternative to protecting reproductive value.As we move into the post-modern time period, new definitions of sustainability will attempt to incorporate he economic and social aspects of fisheries and/or ecosystem and habitat requirements. These definitions now involve "warm and fuzzy" notions (healthy ecosystems and fishing communities, the needs of future generations, diverse fish communities) and value judgements of desired outcomes. Additional work is needed to make these definitions operational and to specify quantitative objectives to be achieved. In addition, multiple objectives may be incompatible, so trade-offs in what constitutes sustainability must be made. The advances made under the single-species approach should not be abandoned in the post-modern era, but rather enhanced and combined with new approaches in the multi-species and economic realms.     

应用年龄结构产量模型评估印度洋黄鳍金枪鱼资源

利用年龄结构产量模型(Age structured production model,ASPM)评估了印度洋黄鳍金枪鱼资源状况,同时结合亲体量-补充量曲线陡度系数和年龄组自然死亡系数的敏感性分析,描述了黄鳍金枪鱼资源的发展趋势、判断了开发状况。研究认为,陡度系数设在0.6-0.8才可能使亲体量产生出最大可持续产量(Maximum sustainable yield,MSY)的水平。采用美洲热带金枪鱼委员会推荐的自然死亡系数值时,评估结果最接近渔业现状。研究发现,随着捕捞努力量的增加,总资源量和亲体量呈逐年下降趋势,但总资源量自1990年后趋向稳定,维持在195.9-263.2万t,平均为221万t;亲体量在1994年后下降到100万t以下,1997年以后处在维持MSY所需亲体量的水平之下,目前仍呈下降趋势。补充量在渔业初期呈现大幅度波动,1978年后趋于稳定,并维持在3258.36-6583.35×106尾,平均为4687.66×106尾。未成熟鱼的数量总体较为稳定,但成熟鱼的数量出现剧减,从渔业初期的246.51×106尾减少到2005年的19.02×106尾。模型估计的总捕捞死亡系数从渔业初期开始逐渐上升,1991年后出现大幅度上升,处于0.334-0.456间,2003年时超过FMSY,捕捞产量也于2003年超过MSY。分析认为,2003年以来印度洋黄鳍金枪鱼的持续高产量被认为是不可持续,根据ASPM估算,2003-2006年均产量46.4万t,超过了MSY(36.4万t);S/SMSY为0.76;Fall/FMSY为1.39,由此判断现阶段印度洋黄鳍金枪鱼正处于过度捕捞状态。     

On the evolution of non-specific mutualism

It has been argued that mutualisms are non-specific when mutualistic interactions are weak and transient, and become more specific as interactions increase in strength. However, this runs counter to the observation that there exist tightly linked mutualisms of great antiquity that are highly nonspecific. Here we argue that mutualism generates positive, interspecific, frequency-dependent selection, which acts as a cohesive evolutionary force, discouraging evolution of specificity. A simple mathematical model is constructed to analyse the evolution of a community consisting of two guilds of species with mutualistic between-guild interactions, two competing species in each guild and two genetically distinct phenotypes within each species. With some simplifying assumptions, the trajectories in the neighbourhood of the only interior equilibrium point are determined analytically in terms of interactions between individuals. These show that the equilibrium is locally stable (no evolution) when there is little differentiation between phenotypes in mutualistic and interspecific, competitive interactions. On the other hand, when there is strong differentiation between phenotypes in their mutualistic interactions, the equilibrium is unstable and the community starts to evolve towards non-specificity. There are, however, two forces counteracting this tendency which, if sufficiently potent, cause evolution towards specificity. The first is generated by strong differentiation between phenotypes in interspecific competition; the second is caused by specificity which already exists between species in their mutualistic interactions. Thus, the tendency for non-specificity or specificity to evolve depends on the interplay between antagonistic and mutualistic interactions in the community. We illustrate these results with some numerical examples and, finally, survey some data on specificity of mutualisms in the light of the analysis.     

基于栖息地指数的东太平洋黄鳍金枪鱼渔场预报

黄鳍金枪鱼是东太平洋海域重要的金枪鱼种类之一,也是我国金枪鱼延绳钓的主要捕捞对象之一。根据2011年东太平洋海域(20°N—35°S、85°W—155°W)延绳钓生产统计数据,结合表温(SST)和海面高度(SSH)的遥感数据,采用频次分析法获得黄鳍金枪鱼分布的SST和SSH适宜范围;运用一元非线性回归方法,以渔获量为适应性指数,按季度分别建立了基于SST和SSH的长鳍金枪鱼栖息地适应性指数,采用算术平均法获得基于SST和SSH环境因子的栖息地指数综合模型,并用2012年各月实际作业渔场进行验证。结果显示,黄鳍金枪鱼渔场多分布在SST为24—29℃、SSH为0.3—0.7 m的海域。采用一元非线性回归建立的各因子适应性指数模型在统计上均为显著(P0.05)。经与2012年实际生产情况比较,作业渔场预报准确性达66%以上。研究获得栖息地指数模型可为金枪鱼延绳钓渔船寻找中心渔场提供参考。     

Model analysis of the effect of environmental fluctuation on the species replacement pattern of pelagic fishes under interspecific competition

There are two factors affecting long-term fluctuation of planktotrophic pelagic fish: environmental fluctuation and interspecific competition. Long-term catch data of planktotrophic pelagic fishes in Japan suggest that the chub mackerel (species B) was replaced by the sardine (A), A was replaced by the anchovy, Pacific saury and horse mackerel (Group C), and species in group C were replaced by species B. If species A defeats B, B defeats C, and C defeats A in interspecific competitive ability, then the abundance of these three groups fluctuate forever and dominate in the same order. We call this cyclic advantage hypothesis for species replacement. In this model, environmental fluctuation affects the species replacement as a trigger. Environmental fluctuation does not determine the next dominant species but greatly affects when the next replacement occurs.     

Size‐selective fishing affects sex ratios and the opportunity for sexual selection in Alaskan sockeye salmon Oncorhynchus nerka

Selective exploitation can cause adverse ecological and evolutionary changes in wild populations and also affect sex ratios but few studies have empirically documented skewed sex ratios in exploited fishes (other than species with extreme sexual size dimorphism, SSD). To investigate the possibility of sex‐selective fishing on Alaskan sockeye salmon Oncorhynchus nerka, we assessed sex ratios in fish at two spatial scales: within each of five fishing districts and among 13 breeding populations in one of these districts. We predicted that populations’ sex ratios would vary based on the average size of fish and SSD because size affects vulnerability to fishing. At the larger scale, we found a small but significant bias in fish returning to four of the five fishing districts (average = 52% females), and in four of the five districts males were caught at significantly higher rates than females. At the finer scale there was marked variation in sex ratio on the breeding grounds, ranging from 36% to 47% males. Populations with fish of intermediate sizes experienced the greatest sex ratio biases; the greater vulnerability of males than females to fishing resulted from a combination of larger SSD and different harvest rates between the sexes associated with the fishery size‐selectivity curve shape. Skewed sex ratios may change competition and behavior on the breeding grounds, relaxing selection on male traits associated with mate choice by females or intra‐sexual competition and altering demographic and evolutionary pressures on the fish. Assessment of the size selectivity of fishing gear and the population's SSD can help to illuminate if and how exploitation can affect sex ratios. Future studies examining size‐selective fishing should also evaluate the consequences for sex ratios, as this might help explain changes in harvested population structure and sustainability.     

Effect of Small Versus Large Clusters of Fish School on the Yield of a Purse-Seine Small Pelagic Fishery Including a Marine Protected Area

We consider a fishery model with two sites: (1) a marine protected area (MPA) where fishing is prohibited and (2) an area where the fish population is harvested. We assume that fish can migrate from MPA to fishing area at a very fast time scale and fish spatial organisation can change from small to large clusters of school at a fast time scale. The growth of the fish population and the catch are assumed to occur at a slow time scale. The complete model is a system of five ordinary differential equations with three time scales. We take advantage of the time scales using aggregation of variables methods to derive a reduced model governing the total fish density and fishing effort at the slow time scale. We analyze this aggregated model and show that under some conditions, there exists an equilibrium corresponding to a sustainable fishery. Our results suggest that in small pelagic fisheries the yield is maximum for a fish population distributed among both small and large clusters of school.     

EVOLUTIONARY RESPONSES TO ENVIRONMENTAL CHANGES: HOW DOES COMPETITION AFFECT ADAPTATION?

The role and importance of ecological interactions for evolutionary responses to environmental changes is to large extent unknown. Here it is shown that interspecific competition may slow down rates of adaptation substantially and fundamentally change patterns of adaptation to long-term environmental changes. In the model investigated here, species compete for resources distributed along an ecological niche space. Environmental change is represented by a slowly moving resource maximum and evolutionary responses of single species are compared with responses of coalitions of two and three competing species. In scenarios with two and three species, species that are favored by increasing resource availability increase in equilibrium population size whereas disfavored species decline in size. Increased competition makes it less favorable for individuals of a disfavored species to occupy a niche close to the maximum and reduces the selection pressure for tracking the moving resource distribution. Individual-based simulations and an analysis using adaptive dynamics show that the combination of weaker selection pressure and reduced population size reduces the evolutionary rate of the disfavored species considerably. If the resource landscape moves stochastically, weak evolutionary responses cause large fluctuations in population size and thereby large extinction risk for competing species, whereas a single species subject to the same environmental variability may track the resource maximum closely and maintain a much more stable population size. Other studies have shown that competitive interactions may amplify changes in mean population sizes due to environmental changes and thereby increase extinction risks. This study accentuates the harmful role of competitive interactions by illustrating that they may also decrease rates of adaptation. The slowdown in evolutionary rates caused by competition may also contribute to explain low rates of morphological change in spite of large environmental fluctuations found in fossil records.     

Fish by‐catch in shrimp beam trawls in the northern South China Sea

The by‐catch of shrimp beam trawl fisheries in the northern South China Sea were estimated during the 2005–2006 and 2012–2013 fishing seasons. A total of 98 hauls from three important shrimp fishing grounds were used in the analyses. A total of 119 by‐catch species were captured, compared with only 15 target species. The ratio of by‐catch to shrimp ranged from 1.01 to 13.90. The dominant standard length (SL) range of fish by‐catch was 51–100 mm. The percentage of juveniles in the total catch of some fish species was high (e.g. 100% for A. aneus).     

Towards a theory of insect epidemiology

An attempt is made to consolidate and extend some of our current thoughts on insect epidemiology using graphical reproduction models. Starting with a simple model with a single equilibrium point, the elementary hypothesis is proposed that epidemics erupt when this equilibrium point increases substantially through improvement of the insect's habitat. The extension of this model to more than one coincident equilibria, some of which may be locally stable, is discussed and generalized using the theory of habitat suitability. Use of equilibrium manifolds is suggested to permit greater dimensionality. Lastly, an explanation of insect epidemics, based on the effects of time delays in the response of density-dependent processes, is elaborated and generalized. The influence of spatial dimensions and insect dispersal on the theory is discussed.