Fishing Mortality Reference Points Research Articles

Evaluating methods for estimating shark natural mortality rate and management reference points using life‐history parameters

AbstractSharks (class Chondrichthyes, subclass Elasmobranchii) typically have a long lifespan, slow growth rate and low fecundity, leading to low productivity and hence relatively high vulnerability to fishing. Information for managing fisheries catches of elasmobranchs is often lacking, because elasmobranchs are usually non‐target species, often with low abundance. It is more feasible to develop management reference points for elasmobranchs based on their life‐history information than through traditional stock assessments. The natural mortality rate (M) is the leading life‐history parameter (LHP) required by many methods for developing reference points and is itself often derived indirectly from other LHPs. In this paper, we evaluate nineMestimators, using 15 shark stocks in the Western and Central Pacific Ocean as examples. We then compare four methods for developing fishing mortality reference pointsFmsy, including empirical modelling, the Euler‐Lotka equation, anM‐productivity‐based method and the spawning potential rate converted method. Our analyses show that popularMestimators developed mainly from teleost data resulted in large deviations from the average and were not suitable for elasmobranchs. All four methods for estimatingFmsyperformed similarly. However, the empirical method is very simple and cost‐effective and tended to produce smaller deviations from the average than the other three methods. Nonetheless, it is recommended that multiple methods should be used to minimize possible bias and reflect uncertainty, if the required LHPs are available.

Setting biological reference points for sea scallops (Placopecten magellanicus) allowing for the spatial distribution of productivity and fishing effort

Management for the major sea scallop (Placopecten magellanicus) fisheries in Canada is based on maximum sustainable yield (MSY) biomass and fishing mortality reference points applied to the whole stock, under the assumption that fishing mortality is uniformly distributed in space. However, scallop fishing vessels concentrate fishing in areas that consistently exhibit high densities resulting in a nonuniform spatial distribution of fishing effort. This study applies a spatial model for fishing effort derived from satellite vessel monitoring system data, scallop habitat suitability maps, and relative scallop density from a spatial stock assessment model to evaluate precautionary approach reference points in support of sustainable management. Target harvest rates were evaluated in terms of MSY for the higher habitat suitability areas. The results indicated that although MSY for the spatial model were similar to those when assuming a uniform distribution of effort, the biomass and catch rates over all areas were higher. The spatial model predicted that the MSY would be taken with less fishing effort, potentially lessening the benthic impacts from the scallop fishery.

Local sensitivity of per-recruit fishing mortality reference points

ABSTRACTWe study the sensitivity of fishery management per-recruit harvest rates which may be part of a quantitative harvest strategy designed to achieve some objective for catch or population size. We use a local influence sensitivity analysis to derive equations that describe how these reference harvest rates are affected by perturbations to productivity processes. These equations give a basic theoretical understanding of sensitivity that can be used to predict what the likely impacts of future changes in productivity will be. Our results indicate that per-recruit reference harvest rates are more sensitive to perturbations when the equilibrium catch or population size per recruit, as functions of the harvest rate, have less curvature near the reference point. Overall our results suggest that per recruit reference points will, with some exceptions, usually increase if (1) growth rates increase, (2) natural mortality rates increase, or (3) fishery selectivity increases to an older age.

Updated Maturity Estimates for Flatfishes (Pleuronectidae) in the Eastern Bering Sea, with Implications for Fisheries Management

AbstractFemale Yellowfin Sole Limanda aspera, Alaska Plaice Pleuronectes quadrituberculatus, and Flathead Sole Hippoglossoides elassodon were collected from the eastern Bering Sea during known prespawning, spawning, and postspawning periods in 2012 and 2013, and their ovaries and otoliths were sampled for use in histological analysis to update historical maturity estimates. For fisheries management, new information on maturity at age can lead to possible changes in estimated reproduction potential (measured as female spawning stock biomass [SSB]) and values of fishing mortality reference points. Our analysis indicated that Yellowfin Sole currently mature at an age similar to that estimated in a study conducted 20 years ago. An evaluation of impacts on the stock assessment indicated that updated estimates of Yellowfin Sole SSB were over 7% higher, but the reference points only changed slightly. The first histologically derived maturity estimates for Alaska Plaice were close to the anatomically derived estimates (visual assessments from 1987), resulting in a marginal decrease (5%) in SSB, but changes in reference points were near 10%. Based on the new maturity estimates for Flathead Sole, SSB estimates increased by 7% compared with estimates currently used in the stock assessment, which relied on maturity data collected in 1999 and 2000. The change in Flathead Sole SSB was concomitant with changes of 16–18% in fishing mortality reference points. Our results indicated minimal differences from historical maturity estimates after re‐examination, but in some cases those differences led to relatively large changes in the respective reference points, underscoring the reference points' sensitivity to changes in maturity. Incorporation of these new maturity estimates into the stock assessment process provides valuable updated information for fisheries managers. However, a more comprehensive sampling program is needed to investigate the spatial and temporal aspects of reproduction for each species.Received November 13, 2014; accepted September 3, 2015

An evaluation of fishing mortality reference points under varying levels of population productivity in three Atlantic cod (Gadus morhua) stocks

Variation in productivity will affect the level of fishing mortality that a population can sustain without decline. We examined three Atlantic cod (Gadus morhua) stocks off Canada for evidence of changing productivity and determined the impact this variation would have on different fishing mortality reference points and their sustainability. Productivity was found to vary greatly over time within all three cod stocks. Under high productivity conditions, G0 (i.e. the potential growth in spawning-stock biomass at a fishing mortality of zero) for the three populations was 20–30%. But under low productivity conditions, G0 was much less. Two of the populations had G0 that was near zero or negative when productivity was low, indicating the possibility of population decline even in the absence of fishing. The degree to which the levels of common fishing mortality reference points (FMSY, FMAX, F0.1, and F40%SPR) changed across productivity periods was variable. All showed significant variation with changing productivity; however, the differences in reference points between productivity periods were generally very small except for FMAX and FMSY. All four reference points examined here were sustainable under conditions of high and average productivity. YPR and SPR reference points do not incorporate recruitment in their calculation. During periods of low productivity, recruitment was reduced and these reference points generally became unsustainable. FMAX was similar to FMSY only under high and average productivity but was not a good proxy for FMSY under lower levels of productivity. Reference points should incorporate recruitment because of its importance in determining the productivity of the stock and should be updated as productivity changes.

Varying components of productivity and their impact on fishing mortality reference points for Grand Bank Atlantic cod and American plaice

Population productivity is determined in large part by growth, maturity and recruitment. This determines the level of fishing that the population can sustain without declining. We examined how these components of productivity have varied during warm and cold periods for two Grand Bank fish populations, and how this variation affects fishing mortality reference points. Productivity of both Div. 3NO Atlantic cod and Div. 3LNO American plaice has varied considerably over time. Projections of population size under different conditions showed that the level of recruits per spawner played a major role in determining the level of fishing mortality that did not result in population decline. For plaice there was also a substantial impact of varying proportion mature at age. The impact of factors other than temperature on recruits per spawner and maturity meant that the association of productivity with temperature was not consistent. When productivity was at its lowest, the level of fishing mortality that could be sustained without causing rapid population decline was very low. The results of this study clearly demonstrate that the impacts of changing productivity can be rapid and very large and if fishing mortality reference points are not adjusted accordingly the results can be catastrophic. It may be helpful to develop fishing mortality reference points that incorporate all components of productivity and are updated using only recent data. Such reference points could be used in combination with buffers around a biomass limit reference point (Blim) and/or low risk tolerances for declining below Blim to account for the uncertainties that would remain even in frequently updated reference points.

Sustainable Exploitation of Fish Stocks by Long-Term Profit Maximization with Special Regards to Cod in the Baltic Sea

An important aim of fishery management is the conservation and sustainable exploitation of fish stocks whereas maximization of profit is the aim of fishermen. At first view this seems to be contradictory. The main management tools for stock exploitation are fishing effort (summarizing all effort limitations) and the so called technical measures (mesh size). The presented new model, using these control variables, is intended to replace the Gordon Schaefer model and based on the Beverton and Holt yield-per-recruit model. The Beverton and Holt model, however, do not take into account the feedback of stock-size changes on recruitment and therefore also on yield. Overcoming this disadvantage is one finding in this paper. For the computation of optimal exploitation pattern the Shepherd approach for construction of sustainable yield curves was extended by incorporating the control of mesh size along with the current exclusive control of fishing effort. This extension turns all fishing mortality reference points into reference functions of the age at first capture. The sole economic important demersal stock in the western Baltic Sea is the cod stock of that area. This paper describes an equilibrium model for the computation of long-term harvesting strategies for that stock including the control of mesh size into the well-established exploitation models. It was shown that the stock is more reliably protected against collapsing by economic considerations rather than by measures chosen according to the precautionary approach. If the Baltic cod stock is exploited according to the suggested approach with an optimized, much larger mesh size in the steady-state, a larger profit is ensured simultaneously with improved stock conservation. This model can also be applied for the maximization of the overall profit within mixed-species management by overlaying the single-species results. Here, however, it is only possible to find an overall optimum which generally differs from the single species optimum. An economic reasonable limit of exploitation is given by the FMSY-curve – the fishing mortality resulting for a given age at first capture in the maximum yield for this age while the Fpa-curve according to precautionary approach has to be considered as the final line of defense against stock collapse. The precautionary approach only calls for changes in effort limitations or technical measures if a violation of its principles has occurred in a multi-species fishery due to ecological reasons. Apart from stock protection the main objective of this paper is to find at the same time appropriate values for the management control variables defining an equilibrium maximizing the profit (maximum economic yield – MEY) of the fishery.

Reproductive Biology of Pacific Ocean Perch and Northern Rockfish in the Aleutian Islands

AbstractEstimates of maturity (i.e., the proportion of mature individuals at age) play an important role in assessing stock productivity; however, this information has not been published for rockfishes in the Aleutian Islands. Ovary and otolith samples were collected from Pacific Ocean Perch Sebastes alutus and Northern Rockfish S. polyspinis during both fishery‐independent and fishery‐dependent cruises in 2010 to investigate maturity and other aspects of reproductive biology. Histological examination of ovaries indicated that both species exhibited general parturition in April, while specimens caught in the summer and fall were maturing and generally exhibited progressive stages of vitellogenesis. Histological analysis was more reliable when characterizing ovary condition than were macroscopic or visual assessments during the developing period. Estimates of natural mortality, based on a relationship with the gonadosomatic index, were 0.040 for Pacific Ocean Perch and 0.042 for Northern Rockfish. Mean fecundity estimates calculated for Northern Rockfish (89,320 oocytes) were correlated more with length and weight than age. Estimates of age and length at 50% maturity were 9.1 years at 32.4 cm for Pacific Ocean Perch and 7.6 years at 27.7 cm for Northern Rockfish and indicate maturation at younger ages than do previous studies from the Gulf of Alaska. This information on reproductive biology will improve estimation of fishing mortality reference points and management of these commercially important species in the Aleutian Islands.Received June 22, 2012; accepted December 10, 2012

The estimation and robustness of FMSY and alternative fishing mortality reference points associated with high long-term yield

Equations for equilibrium yield and biomass are presented and used to derive FMSY and alternative reference points (ARPs). ARPs are analogues of the traditional points based on yield-per-recruit (YPR) or spawning stock-per-recruit (SPR) (F0.1, F40%, F50%), but refer to the equilibrium total yield or biomass. The method combines YPR and SPR analysis with stock–recruitment relationships. The sensitivity of FMSY and ARPs to the range of available stock–recruitment data, recruitment variance, various steepness levels in the stock–recruitment models, assessment variance, and bias are tested. The analysis showed that in most cases, F40%B and F50%B, defined by the equilibrium biomass (B), were the most robust relative to the different sources of uncertainty. However, in the case of the misspecification of the stock–recruitment relationship, FMSY showed superior performance. F40%B for the Beverton and Holt stock–recruitment model and F50%B for the Ricker recruitment model can be recommended as conservative fishing mortalities associated with high long-term yield. For the considered steepness, they produced yield up to 5%–10% lower than yield at FMSY.

Failing the high seas: A global evaluation of regional fisheries management organizations

Abstract Regional fisheries management organizations (RFMOs) collectively manage the largest distinct area of the world, the high seas, but their effectiveness in conserving the fish stocks therein has been questioned lately, as many stocks have declined. This study quantitatively assesses the effectiveness of the world's 18 RFMOs, based on a two-tiered approach, concentrating first on their performance ‘on paper’ and secondly, in practice. The former was determined by assessing how well RFMOs scored against 26 criteria that together reflect current RFMO best practices. The latter assessment referenced the current state of the stocks RFMOs manage, through biomass and fishing mortality reference points and biomass trends through time. Results show low performance of RFMOs for both assessments, i.e., average scores of 57% and 49%, respectively. The latter result is emphasized by findings that reflect two-thirds of stocks fished on the high seas and under RFMO management are either depleted or overexploited. Findings also indicate that there is no connection between the two sets of scores, suggesting a disparity between organization intent and action.

Can economic and biological management objectives be achieved by the use of MSY-based reference points? A North Sea plaice (Pleuronectes platessa) and sole (Solea solea) case study

AbstractPilling, G. M., Kell, L. T., Hutton, T., Bromley, P. J., Tidd, A. N., and Bolle, L. J. 2008. Can economic and biological management objectives be achieved by the use of MSY-based reference points? A North Sea plaice (Pleuronectes platessa) and sole (Solea solea) case study. – ICES Journal of Marine Science, 65: 1069–1080. We examined the biological and economic impact of changing from management based on single-species limit reference points to one based on alternative targets, using the multispecies multifleet North Sea flatfish fishery. The robustness of reference points was tested against identified changes in plaice and sole biology. Current ICES single-species limit and precautionary biomass and fishing mortality reference points were seldom consistent with each other. Although they were generally robust to biological uncertainty, fishing at Fpa for sole could lead to stock collapse under one biological scenario. Adoption of alternative targets would reduce reliance on current reference points as stocks moved to a more sustainable state. Maximum sustainable yield (MSY), maximum economic yield (MEY), and maximum employment conditions implied different effort levels in the two fleets modelled, and different profits. Ftarget could be achieved with equal effort reductions in both fleets. Changes in stock biology affected the fishing effort required to maximize employment within the fishery, whereas MSY, Fmax, and MEY targets were robust to this uncertainty. Resulting profits and yields did vary widely, however. The selection of target reference points therefore requires stakeholders to define fishery objectives explicitly, against which targets can be evaluated for the resulting trade-offs between risk to stocks, yield, employment, and other social objectives.

Evaluation of multiple management objectives for Northeast Atlantic flatfish stocks: sustainability vs. stability of yield

Abstract This paper describes a simulation study that evaluated the ICES scientific advisory process used to recommend total allowable catches (TACs) for flatfish stocks. Particular emphasis is given to examining the effects on stock biomass, yield and stability of constraining interannual variation in TACs. A “management strategy evaluation” approach is used where an operating model is used to represent the underlying reality, and pseudo data are generated for use within a management procedure. The management procedure comprises a stock assessment that uses data to estimate parameters of interest and a decision rule to derive TAC recommendations for the following year. Bounds on TAC of between 20% and 40% have little effect on yields or stability, while a 10% bound on TAC can affect the ability to achieve management targets and result in low-frequency cycling in the stock. In the short term, performance is highly dependent on current stock status but bounds have less effect if the stock is close to equilibrium for a target fishing mortality (F). In addition, it was shown that current ICES biomass and fishing mortality reference points are not always consistent, and several are clearly inappropriate. Importantly, including realistic sources and levels of uncertainty can result in far from optimal management outcomes based on the current procedures. Results also conflicted with expert opinion, in suggesting that management based on a fixed F regime could result in relatively stable yields despite fluctuations in year-class strength and that the management feedback process itself is implicated in causing fluctuations in the system due to significant time-lags in this process. We therefore emphasize that providing more precise population estimates or developing harvest control rules alone will not necessarily help in achieving management objectives, rather management procedures that are robust to uncertainty and tuned to meet management objectives need to be developed. Operating models in these simulations were constrained to be based on existing ICES methods and perceptions of stock dynamics, but we recommend that, in future, operating models that represent the best available understanding of the actual system dynamics be used to evaluate models and rules considered for application.

An evaluation of the implicit management procedure used for some ICES roundfish stocks

Abstract This paper describes a simulation study that evaluated the performance of the scientific advisory process used by ICES to recommend total allowable catches (TACs) for roundfish stocks. A “management strategy evaluation” approach is used, involving development of an operating model to represent the underlying reality, and an observation model to generate pseudo data that are then used within a management procedure. The management procedure comprises an assessment that uses data to estimate parameters of interest and a decision rule to derive TAC recommendations for the following year. There are two important results: including realistic sources and levels of uncertainty can result in far from optimal management outcomes based on the current procedures; and current ICES biomass and fishing mortality reference points are not always consistent, and several are clearly inappropriate. This is because the types of projection used by ICES do not incorporate important lags between assessing stock status and implementing management measures, and they also ignore important sources of uncertainty about the actual dynamics, as well as our ability to collect data and implement management regulations (i.e. model, measurement, and implementation error, respectively). The simulation approach also showed that better management is not necessarily going to be achieved by improving the assessment, because even with a perfect assessment (where the simulated working group knew stock status perfectly), stocks may crash at fishing levels that standard stochastic projections would suggest were safe. It is proposed that, in future, operating models that represent the best available understanding of the actual system dynamics be used to evaluate models and rules considered for application. These operating models should capture the plausible range of characteristics of the underlying dynamics, but not necessarily model their full complexity. In general, they will be more complex than those used by assessment working groups, so developing management procedures that are robust to a broad range of uncertainty. However, the models and rules used as part of the management procedure should be simpler than those used at present.

Impacts of demographic variation in spawning characteristics on reference points for fishery management

Parametric relationships of recruitment to an index of parental stock size assume the latter is proportionally related to spawning potential, irrespective of the demographic composition of adults. Recent empirical information, however, suggests that reproduction by older and experienced females is more successful than by young and inexperienced females. New models are developed incorporating the proportion of each age group spawning for the first, second, etc., time (from information contained in the maturity ogive) and differences in the survival of eggs and larvae in relation to the demography of spawners (based on experimental results). A series of spawning metrics [spawning-stock biomass (SSB), egg production, hatched egg production, viable larval production] and associated recruitment-based fishing mortality reference points (Fmed, Fcrash, and the F that allows at least one lifetime spawning per recruit) are contrasted for the Georges Bank Atlantic cod (Gadus morhua) stock. The time-series of spawning intensity is significantly altered when hatched eggs or viable larvae are used as the metric, reflecting the contribution of first- and second-time spawners in some years, and the increasing trend in F. The effect of reduced average maternal size in relation to egg viability was a more important factor contributing to discrepancies with SSB than was spawning experience in relation to hatching success. Percent maximum spawning potential (%MSP) per recruit curves in relation to F are steeper (i.e. result in lower values of %MSP for a given F) for hatched eggs and viable larvae than for SSB or egg production per recruit. Lifetime expected numbers of spawnings per recruit are significantly reduced when the effects of spawning experience on egg hatching success are included. And finally, although point estimates of Fmed and Fcrash are similar using SSB and viable larval production, the median Fcrash estimated from 5000 bootstrap realizations of the Beverton-Holt stock recruitment curve for viable larvae is much lower than that from SSB, with narrower confidence bounds. Our results suggest that traditional approaches to F-based reference points using SSB systematically overestimate the resiliency of stocks to fishing. If age-at-spawning is at least partially heritable, then intense fishing on younger ages may exert high levels of selection for early maturity with negative impacts on net reproductive effort and trait diversity.

Population dynamics of rock sole ( Lepidopsetta bilineata) in the North Pacific

We compared the population dynamics of two geographically discrete stocks of rock sole ( Lepidopsetta bilineata) one in the Bering Sea off Alaska near the centre of the geographic range and the other in Hecate Strait off British Columbia near the southern limit of the species’ range. Rock sole in Hecate Strait had a higher rate of growth and natural mortality, matured earlier and had a higher reproductive potential than rock sole in the Bering Sea. The difference in the temperature regime probably contributes to the slower growth rate and longer time to maturity for the Bering Sea stock. There was evidence of density dependence in the relationship between spawners and recruits mitigated by ocean temperature for both stocks. Productivity declined at high stock sizes. We suggest that, in the case of rock sole in Hecate Strait density dependence could result from competition for food and habitat at the larval and juvenile stages. The species exhibits highly localised spawning which results in increased competition for a very limited food resource and prime habitat. In the case of Bering Sea rock sole production at high stock densities was impaired more than it was in Hecate Strait. This may have severe long-term consequences especially if environmental conditions are unfavourable for recruitment. We examined the effect of temperature at the time of spawning on subsequent recruitment for both stocks. Plots of temperature recorded at the time of egg and larval development vs. subsequent recruitment in the Bering Sea did not show a discernible relationship, although it was a significant component in a Ricker formulation of spawner-recruit analysis. Temperature had a moderating influence on rock sole year-class strength in Hecate Strait but could not be considered a controlling factor. There was no synchrony in year-class strength between the two stocks suggesting that the two stocks are responding differently to environmental forcing factors and/or differences in spawning stock biomass. Target fishing mortality reference points for F 0.1 and F med were higher for the rock sole stock in Hecate Strait than for the Bering Sea stock. Similarly, higher yield was associated with a given rate of fishing mortality for the Hecate Strait stock compared to the Bering Sea stock. A fixed target fishing mortality reference point that is sustainable for the Hecate Strait stock could result in an overfishing situation and a significant decline in abundance for the Bering Sea stock. The highest sustainable yield occurred at a fishing rate slightly above F 0.1 for both stocks.

The importance of environmental factors in the design of management procedures

The effects on recruitment of environmental factors such as temperature have been studied for decades. Correlations between environmental and biological indices have arisen, but have then broken down. In some cases, mechanisms that explain a strong correlation have been identified and management procedures to account for them have been developed. There are more such examples in crustacean and pelagic fisheries than in demersal fisheries. In this study, a Monte Carlo simulation approach to explore the implications of ignoring the effect of environmental factors on recruitment and on setting fishing mortality reference points is described. A gadoid-like example, with three levels of strength of interaction between the environmental factor and recruitment, is considered. In these examples, there is no gain (in terms of either conservation or average yield) when an environmental factor is incorporated in the short-term prediction of recruitment. Changes in fishing mortality reference points only lead to gains when the environmental factor can be well predicted. The importance of simulation studies in exploring the likely benefits and feasibility of incorporating environmental factors in management procedures is emphasized.