Constant Fishing Mortality Research Articles

A simulation-based option to assess data-limited fisheries off West African waters

Most sophisticated stock assessment models often need a large amount of data to assess fish stocks, yet this data is often lacking for most fisheries worldwide, resulting in the increasing demand for data-limited stock assessment methods. To estimate fish stock status, one class of these data-limited methods uses simply catch time series data and, in other instances, life history information or fishery characteristics. These catch-only methods (COMs) built differently are known to make assumptions about changes in fishing effort and may perform differently under various fishing scenarios. As a case study, this paper used European anchovy (Engraulis encrasicolus) caught in the northwest African waters, though very economically and ecologically important, but still unassessed. Our study investigated the performance of five COMs under different fishing scenarios using as a reference the life-history information of the European anchovy captured in this region of the Atlantic. Hence, the present study developed a simulation approach to evaluate the performance of the five COMs in inferring the stock biomass status (B/BMSY) with consideration of different fishing scenarios under prior information true to anchovy. All five COMs mostly underestimated B/BMSY throughout the simulation period, especially under constant fishing mortality, and in the last five years of the simulation during all fishing scenarios. Overall, these COMs were generally poor classifiers of stock status, however, the state-space COM (SSCOM) generally performed better than the other COMs as it showed possibilities of recovering an overfished stock. When these methods were explored using actual anchovy catch data collected in the northwest African waters, SSCOM yielded results that were deferred from the other COMs. This study being the first to assess this species’ stock in this area using a suite of COMs, presents more insights into the species stock status, and what needs to be considered before scientifically putting in place management measures of the stock in the area.

Optimization of Age-Structured Bioeconomic Model: Recruitment, Weight Gain and Environmental Effects

More and more fishery researchers begin to acknowledge that one-dimensional biomass models may omit key information when generating management guidelines. For the more complicated age-structured models, numerous parameters require a proper estimation or a reasonable assumption. In this paper, the effects of recruitment patterns and environmental impacts on the optimal exploitation of a fish population are investigated. Based on a discrete-time age-structured bioeconomic model of Northeast Atlantic mackerel, we introduce the mechanisms that generate 6 scenarios of the problem. Using the simplest scenario, optimizations are conducted under 8 different parameter combinations. Then, the problem is solved for each scenario and simulations are conducted with constant fishing mortalities. It is found that a higher environmental volatility leads to more net profits but with a lower probability of achieving the mean values. Any parameter combination that favours the older fish tends to lend itself to pulse fishing pattern. The simulations indicate that a constant fishing mortality around 0.06 performs the best. A comparison between the optimal and the historical harvest shows that for more than 70% of the time, the optimal exploitation precedes the historical one, leading to 43% higher net profit and 34% lower fishing cost.

Bioeconomic modelling of grey seal predation impacts on the West of Scotland demersal fisheries

Abstract The role grey seals have played in the performance of fisheries is controversial and a cause of much debate between fishers and conservationists. Most studies focus on the effects of seal damage to gears or fish and on prey population abundance but little attention is given to the consequences of the latter for the fisheries. We develop a model that quantifies the economic impact of grey seal predation on the West of Scotland demersal fisheries that traditionally targeted cod, haddock and whiting. Three contrasting fishing strategy scenarios are examined to assess impacts on equilibrium fleet revenues under different levels of seal predation. These include status quo fishing mortality (SQF, steady state with constant fishing mortality), open access fishing (bioeconomic equilibrium, BE) and the maximum economic yield (MEY). In all scenarios, cod emerges as the key stock. Large whitefish trawlers are most sensitive to seal predation due to their higher cod revenues but seal impacts are minor at the aggregate fishery level. Scenarios that consider dynamic fleet behaviour also show the greatest effects of seal predation. Results are sensitive to the choice of seal foraging model where a type II functional response increases sensitivity to seal predation. The cost to the fishery for each seal is estimated.

Overlooked biological and economic implications of within-season fishery dynamics

Catch equations relate fisheries catch to initial fish abundance and the applied fishing pressure. The Baranov catch equation, often simply referred to as the catch equation, is the commonest one. However, there are exactly three ways of describing seasonal progression of fishing parsimoniously with a single parameter: assume catch rate, fishing effort, or fishing mortality is constant, the last being the assumption underlying the Baranov catch equation. These assumptions imply different dynamics, and only in special cases two of these assumptions can hold true simultaneously. Whether this happens is dictated by the concentration profile (i.e., the dependence of mean fish density where fishing takes place on total stock abundance). We show that the assumed seasonal progression of fishing and the type of the concentration profile have major implications for fishery dynamics as well as biological and economic consequences of fishing, calling for increased awareness of these overlooked assumptions of fishery dynamics. However, in many cases the Baranov catch equation serves as a good approximation, even when its assumption of constant fishing mortality is violated.

Harvest levels that conserve spawning biomass can provide larger and more stable and sustainable yields in intermixed fisheries

We conducted a simulation-based evaluation of constant fishing mortality control rule performance for four intermixing fish populations patterned after lake whitefish (Coregonus clupeaformis) fisheries in the Great Lakes. Various scenarios consisting of different assumed productivity and mixing levels for the spawning populations were examined to explore how harvest level performance changed under conditions that lake whitefish, as well as other fish populations, may encounter. The simulation framework included an operating model that represented the true dynamics of the populations, the harvests from fisheries exploiting the intermixed populations, and a full age-structured assessment of each exploited stock. Four harvest levels were evaluated: a 65% total annual mortality harvest level that presently is used to manage lake whitefish fisheries in northern lakes Michigan and Huron and three more conservative levels (35, 45, and 55% total annual mortality harvest levels). Under many investigated scenarios, the current 65% harvest level was found to have a high risk of overexploiting lower productivity spawning populations, which reduced aggregate yield for the modeled system. The 55 and 45% harvest levels resulted in the largest aggregate yields for many of the examined scenarios, although the 45% harvest level was better at reducing risks to sustainability of individual spawning populations. Inter-annual variation in yield generally declined as harvest levels became more conservative. Our analysis suggests that harvest levels that conserve spawning biomass can provide multiple benefits for intermixed fisheries including greater yields, reduced inter-annual variability in yields, and lower risk of depleting low productivity spawning populations. We encourage the implementation of precautionary harvest rates for intermixed fisheries to help protect less productive populations.

Evaluation of FAD-associated purse seine fishery reduction strategies for bigeye tuna (Thunnus obesus) in the Indian Ocean

In the Indian Ocean, bigeye tuna supports one of the most important fisheries in the world. This fishery mainly consists of two components: longline and purse seine fisheries. Evidence of overfishing and stock depletion of bigeye tuna calls for an evaluation of alternative management strategies. Using an age-structured operating model, parameterized with the results derived in a recent stock assessment, we evaluated the effectiveness of applying constant fishing mortality (CF) and quasi-constant fishing mortality (QCF) strategies to reduce fishing effort of purse seining with fish aggregating devices (FADs) at different rates. Three different levels of productivity accounted for the uncertainty in our understanding of stock productivity. The study shows that the results of CF and QCF are similar. Average SSB and catch during simulation years would be higher if fishing mortality of FAD-associated purse seining was reduced rapidly. The banning or rapid reduction of purse seining with FAD resulted in a mean catch, and catch in the last simulation year, higher than that of the base case in which no change was made to the purse seine fishery. This could be caused by growth overfishing by purse seine fisheries with FADs according to the per-recruit analysis. These differences would be more obvious when stock productivity was low. Transferring efforts of FAD-associated purse seining to longline fisheries is also not feasible. Our study suggests that changes are necessary to improve the performance of the current management strategy.

Evaluating alternative harvest policies for yellow perch in southern Lake Michigan

In the southern basin of Lake Michigan, yellow perch ( Perca flavescens) are ecologically and economically important. However, there is no explicit harvest policy for the management of this resource, the authority for which is shared among four U.S. states. We used decision analysis and projections from a stochastic simulation model to aid managers in formulating a harvest policy. In workshops that included management agency personnel and other experts, critical uncertainties relevant to the population (e.g., alternatives for future stock–recruitment relationships and mixing of recruits among management areas) were identified as well as potential harvest policies (using constant fishing mortality or state-dependent control rules) and associated performance statistics. Our simulation model acknowledged uncertainty in the stock–recruitment relationship, parameter uncertainty given such a relationship, stochastic process variation, and uncertainty associated with assessment and implementation errors. We used the model to project age-, sex-, size-, and spatial-dynamics of the yellow perch population, and thus predicted likely distributions of performance statistics for different harvest policies. Performance statistics included time averages of recreational harvest, remaining spawning stock biomass (SSB), and length of harvested fish as well as the frequency of how often such measures were below desirable thresholds. Results indicate that state-dependent policies produce higher average harvests and lower frequency of years with low SSB, but sometimes more frequent years with low harvest, than constant- F policies that lead to similar depletion of average SSB.

Effects of source–sink dynamics on harvest policy performance for yellow perch in southern Lake Michigan

We used a spatially explicit stochastic simulation model to evaluate whether source–sink population dynamics would affect performance of alternative harvest policies for yellow perch, Perca flavescens, in southern Lake Michigan. The model contained four management areas in southern Lake Michigan representing each U.S. state's waters. We parameterized the model such that all recruitment was produced by only one management area, considering each of the four areas to be the sole source in turn, and contrasted results with a base scenario where all areas produced recruits. We evaluated three types of harvest policies: constant- F, where fishing mortality was constant, and two state-dependent policies, where fishing mortality was constant above either 40% or 70% of unfished spawning stock biomass ( B 0) and decreased to 0 at 0% B 0. We used four performance statistics to evaluate polices: (1) average percentage of B 0 remaining, (2) percentage of years with low spawning stock biomass, (3) average recreational harvest, and (4) percentage of years with low recreational harvest. Performance of harvest policies differed predictably depending on which management area was the source because relative productivity of stock-recruitment relationships and growth patterns differed among source areas. Thus, if management areas on the western side of Lake Michigan were the source of most of the recruits, the fishery could support higher fishing mortality rates than if areas on the eastern side of the lake were sources. State-dependent harvest policies were less sensitive to assumptions about the source of recruits than were constant fishing mortality rate policies. The 0–70 policy was most robust to source–sink dynamics across most of the performance statistics and levels of fishing mortality.

A Bioeconomic Analysis of the Norwegian Spring Spawning Herring (NSSH) Stock

A biological model belonging to the Beverton-Holt age-structured family for the Norwegian spring spawning herring (Clupea harengus) (NSSH) is simulated, the outcome of which compares well with actual data on the fishery. This model is then combined with an economic model to help investigate how optimal a management policy of constant fishing mortality will be for a fishery such as the NSSH, which has a highly fluctuating stock biomass. For the range of constant values of fishing mortality explored, and a simulation time horizon of 20 years, a constant fishing mortality of 0.15 turns out to be economically optimal. It should be noted that this result is sensitive to variations in the assumptions underlying key variables of the fishery. For example, when a constant rather than variable recruitment was assumed, a different optimal fishing mortality rate was obtained.

Initial Attempts to Actively Manage Recreational Fishery Harvest in Minnesota

The theories and management style of commercial fisheries management have recently been applied to recreational fisheries. Several Minnesota recreational fisheries are now managed with target harvest levels based on sustainable yield predictors or quotas based on constant fishing mortality rate strategies. Creel limits and length-based regulations are being used to meet established targets or quotas. Three case histories are described. Both benefits and shortcomings have resulted from applying commercial fisheries management programs directly to recreational fisheries. However, recreational fisheries managers may not be controlling total fish harvest. To effectively manage recreational fisheries for “safe satisfaction returns,” fisheries managers may need to adopt conservative, robust harvest regulations, kill quotas, or aggressive regulations on how people fish. The distribution and mix of those options can be determined by good social science, but the efficacy of managing for safe satisfaction returns will depend on good fisheries data (biological and sociological) and great social skill. The lessons learned from the three case histories reviewed could be used to improve recreational fisheries management.

Impacts of density-dependent growth and maturation on assessment advice to rebuild depleted U.S. silver hake (Merluccius bilinearis) stocks

We examined how density-dependent growth and maturation rates would affect scientific advice to rebuild U.S. silver hake (Merluccius bilinearis) stocks to a minimum biomass threshold (SSB50) using an age-structured stochastic projection model. Uncertainty in recruitment and initial stock sizes at age was incorporated to more realistically capture the stochastic nature of the stock's dynamics through time. Performance measures including spawning stock biomass (SSBt), risk (prob(SSBt< SSB50)), and fishery yields were examined under model assumptions of static life history parameters in contrast with density-dependent parameters for various constant fishing mortality rate (F) harvest policies. Model results indicated that unless compensatory mechanisms are accounted for, scientific advice may be too optimistic and lead to unrealistic expectations of F targets to achieve stock rebuilding, requiring longer rebuilding times and entailing more risk and unrealized sustainable yield. For instance, projected median southern stock SSBt exceeded the SSB50 of 104 000 metric tons (t) with 40% risk to the stock by year 20 from harvesting at F = 0.50. However, under the density-dependent assumption, median SSBt did not rebuild as expected, and after 20 years reached only 75 000 t with 85% risk to the stock. Also under this assumption, median yields increased from 9000 to 27 000 t by year 20 but were 30-40% lower than what would have been expected under density independence.

Influence of errors in natural mortality estimates in cohort analysis

The accuracy of the estimation of cohort strength from catch data may be greatly degraded if a poor estimate of the natural mortality rate is entered into the calculation. A straightforward, exact formulation for the error in cohort reconstruction due to a misspecified natural mortality rate is presented. The special case of constant fishing mortality is particularly transparent, allowing the error to be segmented into easily interpreted terms. A change in the fishing mortality may result in a distinct hump in the transient behavior of the bias factor, rather than a simple monotonic adjustment. This implies a similar pattern in estimated cohort strength.

Mixed-Species Yield-per-Recruit Simulations of the Effect of Seasonal Closure on a Central Queensland Coastal Prawn Trawling Ground

An assessment of the effect of seasonal closure on the central Queensland coastal prawn trawling grounds was carried out by mixed-species yield-per-recruit simulations of the fishery based on parameters determined from field studies and analysis of logbook data. Under a conventional scenario of constant fishing mortality, simulations suggest little positive effect on yield to be achieved from closures of up to 15 wk in duration. Economic gain would have to be made via reduced costs during the closure, rather than through increased value of the postclosure yield. Logbook data clearly showed that the fishery underwent a heavy pulse of effort, immediately after the fishing season opened. Nonlocal boats had entered the fishery to take advantage of the prawns available. Using this more realistic regime of fishing mortality, simulations suggest a significant increase in yield and value of catch. The increase was caused by boosted exploitation rates of the stocks and was determined by the magnitude of the effort pulse rather than as a direct benefit of the closure.

An analysis of yield variability from three harvesting strategies in the South African anchovy fishery, under conditions of randomly fluctuating recruitment success

An age-structured population model was used to examine the variability of yield created by randomly fluctuating recruitment success in the South African anchovy Engraulis capensis stock. Three different harvesting strategies were examined: (1) constant age-specific pattern of fishing mortality, (2) constant quota with effort limitation and (3) annual quota adjustments by means of an F msy procedure. Variable recruitment was generated by a stochastic stock-recruit relationship, and mean yields, mean spawner biomasses and probability distributions of yield were calculated at each exploitation level after 150 projections from the population structure of anchovy in 1981. Under conditions of constant fishing mortality, the variability of yield increased continuously as the exploitation rate was increased. The maximum average yield (MAY) is the stochastic equivalent of the maximum sustainable yield (MSY) of deterministic models, but an attempt to sustain this MAY by a constant quota resulted in a very high risk of stock collapse. An Fmsy policy based on pre-season adult biomass resulted in more variable yields than were obtained at the equivalent constant fishing mortality at age. Stability of yield therefore demanded the acceptance of lower average yields than could be attained in the long term by quota adjustment. It was considered that the South African purse-seine industry could not cope with the wide fluctuations of yield necessary to attain MAY. Specific management policies sufficiently robust to withstand both fluctuating recruitment success and the uncertainties in the parameters of working population models would be required.