Georges Bank Haddock Research Articles

Testing of a helix twine off-bottom trawl on Georges Bank

The massive biomass of Eastern Georges Bank haddock (Melanogrammus aeglefinus) is difficult to harvest without capturing less robust, but still valuable groundfish stocks like Atlantic cod (Gadus morhua) and some flatfish species. Specialized haddock trawls that raise the mouth of the nets off-bottom have reduced bycatch but the very poor status of Atlantic cod prioritizes even greater reduction to prevent exceeding regulatory fishing quotas. Raising the entire fishing gear off-bottom may further reduce bycatch while eliminating benthic impacts, expanding access to grounds previously off-limits to bottom-tending trawls. We evaluated an off-bottom trawl (OBT) to harvest Eastern Georges Bank haddock while reducing catches of overexploited stocks. The OBT net has very large meshes at the front end, made with innovative “helix” twine that produces lateral hydraulic forces while towing, resulting in self-spreading of the meshes. We established optimal gear configurations to achieve the target OBT net shape and distance to the seafloor by using an assortment of mensuration sensors/loggers and cameras. The OBT caught similar amounts of haddock and reduced some bycatch more than a standard bottom “Ruhle trawl”, but also caught fish of the same lengths despite the OBT using a smaller mesh-sized codend. The OBT also demonstrated similar requirements in vessel RPMs as the Ruhle trawl, despite having a larger swept area.

Effect of environmental factors and density-dependence on somatic growth of Eastern Georges Bank haddock (Melanogrammus aeglefinus)

Haddock (Melanogrammus aeglefinus) is an important commercial fish species in the Northwest Atlantic, with the Eastern Georges Bank population a transboundary resources utilized by both Canada and the United States. With the dramatic increase recruitment of this population in the last decade, in particular concentrations of fish on the eastern portion of the Bank, there has been a precipitous decrease in somatic growth, raising concerns for managers and stakeholders. In this study, the effect of environmental factors and density-dependence on somatic growth of eastern Georges Bank haddock was studied using a 2-step Generalized Additive Model (GAM) regression approach. The biotic and abiotic factors examined in the regression analysis included haddock density, sampling location, local sea surface and bottom temperature, salinity and short time series of phytoplankton bloom statistics. The relative importance of each covariate in the best selected model was investigated using the nested GAMs. We found that density-dependence explained most of the variation in haddock growth, where the impact of temperature varied by season. Though of less model importance, annual winter salinity and phytoplankton bloom magnitude (lagged by one year) also contributed the growth model fits. These relationships have the immediate application of contextualizing the observed change in haddock growth, but may also inform various aspects of the stock assessment process.

A test of the provisioning hypothesis of recruitment control in Georges Bank haddock

The haddock (Melanogrammus aeglefinus) stock of the Georges Bank region of the US Northeast Continental Shelf displays a pattern of large, episodic recruitments. Among the hypothesized controlling mechanisms is the idea that recruitment events are related to provisioning of prespawning haddock by the fall bloom the year before. With the occurrence of a recent large recruitment event in 2013, it would be prudent to retest this hypothesis. Fall bloom magnitude was positively correlated (r = 0.645, p = 0.005) with haddock survivor ratio (recruits per spawning biomass), including data from the 2013 recruitment. This relationship identifies a pathway of bottom-up control of a resource species, thus focusing concern over recent changes in lower trophic-level productivity.

Interannual differences in larval haddock survival: hypothesis testing with a 3D biophysical model of Georges Bank

AbstractThe ultimate goal of early life studies of fish over the past century has been to better understand recruitment variability. As evident in the Georges Bank haddock (Melanogrammus aeglefinus) population, there is a strong relationship between recruitment success and processes occurring during the planktonic larval stage. This research sought new insights into the mechanisms controlling the recruitment process in fish populations using biological–physical modeling methods together with laboratory and field data sets. We created the first three‐dimensional model of larval haddock on Georges Bank by coupling models of hydrodynamics, lower trophic levels, a single copepod species, and larval haddock. Interactions between feeding, metabolism, growth, vertical behavior, advection, predation, and the physical environment of larval haddock were quantitatively investigated using the coupled models. Particularly, the model was used to compare survival over the larval period and the sources of mortality in 1995 and 1998, 2 years of disparate haddock recruitment. The results of model simulations suggest that the increased egg hatching rates and higher food availability, which reduced starvation and predation, in 1998 contributed to its larger year‐class. Additionally, the inclusion of temperature‐dependent predation rates produced model results that better agreed with observations of the mean hatch date of survivors. The results from this biophysical model imply that food limitation and its related losses to starvation and predation, especially from hatch to 7 mm, may be responsible for interannual variability in recruitment and larval survival outside of the years studied.

Longitudinal Length Back‐Calculations from Otoliths and Scales Differ Systematically in Haddock

AbstractParallel macrostructure analysis of otoliths versus scales was conducted on Georges Bank Haddock Melanogrammus aeglefinus sampled in spring 2011 to test whether annuli widths of both structures yield comparable estimates of back‐calculated length at age. While generally similar in number, scale and otolith annuli were found to differ systematically in their relative widths, mainly because initial annuli are deposited closer to the origin in scales than in otoliths. Accounting for the delayed onset of scale versus otolith formation did not remove the systematic bias. To make both structures comparable, we developed an age‐independent correction approach that can adjust cumulative relative annuli widths (RAWs) of Haddock scales to otolith RAWs by the quadratic formula loge(RAWoto) = 0.7244·loge(RAWscale) + 0.0751·loge(RAWscale)2. Our study comprises a necessary step towards extracting and comparing longitudinal growth data over the entire time series of archived Haddock scales (1931–1993) and otoliths (1994–present) to improve understanding of decadal drivers of growth patterns in this fish stock. Historical longitudinal growth data for exploited stocks are currently underutilized despite their high potential value, given that such data do not confound growth and mortality the way population‐based age–length relationships do. Mixed archives consisting of multiple growth structures exist not only for Haddock but for many commercial fish species; hence, prior to mining these valuable time series of longitudinal data the comparability not only of age but also of back‐calculated growth estimates between structures needs to be ascertained.

Can we increase haddock yield within the constraints of the Magnuson–Stevens Act?

Georges Bank haddock is a recently recovered fish stock in the New England groundfish fishery. Due to federal constraints under the Magnuson–Steven Act, however, this stock cannot be optimally exploited due to the bycatch of other critical species in the New England groundfishery such as cod and yellowtail flounder which are overfished. The Ruhle trawl and Separator trawl are examples of recent advances in gear technology that have been shown to significantly increase haddock to bycatch ratios. This study models the groundfish fishery through a mixed-stock yield model which incorporates technological interactions. We also develop a socio-economic model that quantifies the amount of employment and producer surplus associated with three trawl types. Our results explore policy situations regarding the use of the new trawls. By bridging the biological and socio-economic models, we are able to view the fishery as a system that more accurately represents stakeholder views. Our model shows that each trawl, when used exclusively, produces different optimum strategies and therefore an optimum management strategy would most likely include a combination of trawl types. Our results also support the logic of using modified trawls for haddock fishing trips in which bycatch is strictly regulated (“B days”) as the Ruhle trawl is able to maintain 80% of catches caught by a conventional trawl while reducing bycatch up to over 60%. This paper is a first step towards an aid for policy makers to examine fishery gear trade-offs and the resulting biological and socio-economic consequences of different management actions within the constraints of the Magnuson–Stevens Act.

Comment on “Does the fall phytoplankton bloom control recruitment of Georges Bank haddock, Melanogrammus aeglefinus, through parental condition?”Appears in Can. J. Fish. Aquat. Sci. 65: 1076–1086.

In the paper “Does the fall phytoplankton bloom control recruitment of Georges Bank haddock, Melanogrammus aeglefinus , through parental condition?”, Friedland et al. (Can. J. Fish. Aquat. Sci. 65(6): 1076–1086, 2008) examine a sizable number of hypotheses aiming to explain the recruitment patterns observed in Georges Bank haddock. The authors focus on a correlation between the size of the autumnal phytoplankton bloom and the survivor ratio (recruitment), concluding this to be the main factor determining recruitment, via the mechanism of adult condition at the time of spawning. Here we examine this result in close detail and re-analyse some of the data presented in the paper. We show that the recruitment metric upon which Friedland et al. base their conclusions inadvertently biases the analysis in favour of high recruitment events and against low recruitments. As a consequence, Friedland et al. disregard correlations that are, in fact, significant. Furthermore, we show that the parental condition hypothesis hinges upon a single, highly uncertain data point, without which the correlation is no longer significant. We find that evidence for the parental condition hypothesis is weak, and that in performing the analysis in the chosen manner, Friedland et al. have overlooked alternative hypotheses.

Reply to the comment by Payne et al. on “Does the fall phytoplankton bloom control recruitment of Georges Bank haddock, Melanogrammus aeglefinus, through parental condition?”Appears in Can. J. Fish. Aquat. Sci. 65: 1076–1086.

Payne et al. (Can. J. Fish. Aquat. Sci. 66: 869–872, 2009) raised several points concerning the handling and interpretation of data that went into an analysis of the population dynamics of Georges Bank haddock that suggested a relationship between the fall phytoplankton bloom and recruitment (Can. J. Fish. Aquat. Sci. 65: 1076–1086, 2008). Their main points were the manner in which logarithmic transforms were applied, whether the 2003 year class was truly as large as estimated in a 2006 assessment, and if correlation analyses of zooplankton data should be reconsidered. The reply to these comments was aided by a new assessment which provided additional years of data and improved the quality of the recruitment time series. The reply analyses showed that the relationships were robust to the way the logarithmic transform was applied, the initial estimates of the size of the 2003 year class were correct, and relationships between recruitment and spring zooplankton biomass levels remain statistically insignificant. From these new analyses, the interpretations and conclusions reached in the original paper remain the same; the fall bloom has emerged as a candidate explanatory variable for the stock independent variation in haddock recruitment on Georges Bank.

Differential Egg Mortality of Georges Bank Cod and Haddock Inferred from Two Independent Estimates of Seasonal Egg Production

Georges Bank Atlantic cod (Gadus morhua) and haddock (Melanogrammus aeglefinus) stocks have undergone significant changes over the last 40 years with the reduction of older spawners and increased incidence of young mature fish. Captive studies, from other investigations, have noted that firstand second-time spawners exhibit lower egg hatching success, and especially for haddock compared to cod. Spawning stock biomass (SSB), derived from virtual population analysis (VPA), has been considered as an overestimate of a stock’s spawning potential since it does not fully account for differences in age or size of spawners. The Northeast Fisheries Science Center’s (NEFSC) ichthyoplankton Surveys (1979–1999) have provided an independent estimate of seasonal egg production to compare with a VPA-fecundity based estimate. Since the early stage eggs of the two species are indistinguishable, their numbers were estimated by apportioning the total egg abundance (cod plus haddock) at a station by the late stage abundance ratio, assuming equal mortality for the two species. For cod, the Surveys overestimated the VPA-derived egg production in many years by as much as an order of magnitude. However, for haddock, the VPA-derived egg production estimates were mostly higher than those from the Surveys. Comparing the ratio of the two egg production estimates versus the total combined VPA egg production of cod and haddock, the cod ratios were high when haddock was abundant, especially during the years 1995–1999, and closer to one when cod comprised the greater part of the total. The corresponding analysis for haddock was the reverse to that of cod further supporting the hypothesis that the higher egg mortality of the reduced age population of haddock biased the seasonal egg production estimates of both haddock and cod. These ratios appear to explain 60–70% of the variability in the ratio of Survey to VPA seasonal egg production, while other factors might be related to fecundity and age-class composition of the SSB, or changes in egg mortality rates over the time series. Our Survey estimates also suggest that haddock egg mortality is greater than cod. Thus, the seasonal egg production estimates from egg Surveys based on constant egg mortality are probably inaccurate, especially since the mid-1990s. Cod egg production may have been overestimated and haddock’s underestimated.

Does the fall phytoplankton bloom control recruitment of Georges Bank haddock, Melanogrammus aeglefinus, through parental condition?

In 2003, the Georges Bank stock of haddock ( Melanogrammus aeglefinus ) experienced the largest recruitment event recorded during its assessed history. Several hypotheses have been advanced to explain recruitment variability in this much-scrutinized stock, including variability in the retention of eggs and larvae on Georges Bank, the timing of haddock spawning, and variability in the spring bloom, which influences larval growth and survival. Although these processes may contribute to the formation of successful year classes, none of the factors associated with these previous hypotheses provides an adequate explanation of the 2003 recruitment event. We analyzed data on the dynamics of the fall phytoplankton bloom the year prior to spawning and show it to be highly correlated with subsequent recruitment. We suggest that the fall bloom affects recruitment through enhanced condition of adults and by increasing the quantity and quality of their reproductive output, which in turn leads to a higher probability of survival of their offspring. Although synoptic data on the fall bloom are limited and our analyses are correlative, our purpose is to stimulate a rigorous test of this promising “parental condition hypothesis”.

Redefining the maximum sustainable yield for the Schaefer population model including multiplicative environmental noise

The focus of this article is to investigate the biological reference points, such as the maximum sustainable yield (MSY), in a common Schaefer (logistic) surplus production model in the presence of a multiplicative environmental noise. This type of model is used in fisheries stock assessment as a first-hand tool for biomass modelling. Under the assumption that catches are proportional to the biomass, we derive new conditions on the environmental noise distribution such that stationarity exists and extinction is avoided. We then get new explicit results about the stationary behavior of the biomass distribution for a particular specification of the noise, namely the biomass distribution itself and a redefinition of the MSY and related quantities that now depend on the value of the variance of the noise. Consequently, we obtain a more precise vision of how less optimistic the stochastic version of the MSY can be than the traditionally used (deterministic) MSY. In addition, we give empirical conditions on the error variance to approximate our specific noise by a lognormal noise, the latter being more natural and leading to easier inference in this context. These conditions are mild enough to make the explicit results of this paper valid in a number of practical applications. The outcomes of two case-studies about northwest Atlantic haddock [Spencer, P.D., Collie, J.S., 1997. Effect of nonlinear predation rates on rebuilding the Georges Bank haddock ( Melanogrammus aeglefinus) stock. Can. J. Fish. Aquat. Sci. 54, 2920–2929] and South Atlantic albacore tuna [Millar, R.B., Meyer, R., 2000. Non-linear state space modelling of fisheries biomass dynamics by using Metropolis–Hastings within-Gibbs sampling. Appl. Stat. 49, 327–342] are used to illustrate the impact of our results in bioeconomic terms.

Cod bycatch in otter trawls and in longlines with different bait types in the Georges Bank haddock fishery

In the Northwest Atlantic, bycatch of depleted groundfish stocks in the haddock fishery on Georges Bank is an ongoing concern. In recent years, this fishery on the American side of Georges Bank has been limited by high bycatch rates of cod. Considerable conservation engineering work has been done to address the problem, including experiments with different longline baits and modifications to otter trawl gear. Fabricated baits have shown promise for increased selectivity when longlining, but it has sometimes been difficult to establish whether low cod catch rates with fabricated baits have resulted from bait selectivity or from a low abundance of cod in the area. In this study, we compare catch rates of cod and haddock between otter trawls and longline gear in experimental and commercial fishing on Georges Bank in the summer of 2005. We also compare cod bycatch rates among longlines baited with squid, herring, or fabricated baits (mainly Norbait 700E) in Georges Bank Closed Area 1, from October 2003 to June 2005. Records from the Northeast Fisheries Observer Program were combined with data collected by the Cape Cod Commercial Hook Fishermen's Association (CCCHFA), and compared using generalized linear models. In the Eastern US-Canada Resource Sharing Area (EUSCA) in the summer of 2005 catch of cod per haddock by weight was significantly lower when fishing with longline gear (0.008–0.045 kg cod per kg haddock) than with otter trawl gear (0.059–0.826 kg cod per kg haddock), in all months and areas. Cod bycatch rates were also significantly lower for longlines fishing for haddock than for otter trawls fishing for groundfish species other than haddock. In Closed Area 1, statistically significant differences in cod to haddock ratios were found between baits, with squid catching the highest amount of cod, fabricated baits catching the lowest amount, and herring at an intermediate level. The development of fishing methods that minimize cod bycatch is a high priority in many regions of the North Atlantic. These results indicate that longlines with fabricated bait have the potential to maintain a very low catch of cod while fishing for haddock.

The nascent recovery of the Georges Bank haddock stock

World-wide many fish stocks have been depleted by overfishing. In this study, we describe the nascent recovery of the Georges Bank haddock stock. This mainstay of the New England groundfish fishery was overfished for decades prior to mid-1990s and experienced long-term declines in spawning biomass and recruitment. The stock was considered to have collapsed in the early-1990s when a lawsuit by the Conservation Law Foundation led the New England Fishery Management Council (NEFMC) and the National Marine Fisheries Service (NMFS) to take actions to cease overfishing and to recover Georges Bank haddock and other groundfish stocks. Under restrictive management measures, stock size increased 10-fold from 1995 to 2005. In 2003, an exceptionally abundant year class (YC) was produced. Although this YC may rebuild the haddock stock to pre-1930s abundance if properly fished, monitoring changes in life history parameters and recruitment will be important for sustaining stock recovery. Mean weights and sizes at age of adult haddock have decreased in recent years, and compensatory responses of haddock growth and recruitment to changes in stock density are assessed. We discuss some remaining challenges to managing this recovering transboundary resource in a dynamic multispecies fishery.

Managing fish stocks under climate uncertainty

Abstract The quantitative evaluation of the management of fish stocks under uncertainty requires a formal framework. Decision theory provides that framework. Application of decision theory to fishery management requires information about both the fish stock and the state of the environment. Using Georges Bank haddock as a case study, it is possible to determine the probability of good or poor recruitment using past data and a constant environment. Understanding the state of the environment is more difficult, however, because fixed levels of recruitment, in particular, are associated with different population characteristics, which drastically reduce the sample size for any particular recruitment–environment scenario. Decision theory challenges us to improve our capability of predicting the state of nature, and it appears that this can be accomplished best by reducing the length of the causal chain, a goal now made feasible by the availability of high-resolution, high-frequency ocean models.

Controls on the distribution of Browns Bank juvenile haddock

MEPS Marine Ecology Progress Series Contact the journal Facebook Twitter RSS Mailing List Subscribe to our mailing list via Mailchimp HomeLatest VolumeAbout the JournalEditorsTheme Sections MEPS 263:235-246 (2003) - doi:10.3354/meps263235 Controls on the distribution of Browns Bank juvenile haddock David Brickman* Ocean Sciences Division, Department of Fisheries and Oceans, Bedford Institute of Oceanography, PO Box 1006, Dartmouth, Nova Scotia B2Y 4A2, Canada *Email: brickmand@dfo-mpo.gc.ca ABSTRACT: A combination of biophysical modelling and data analysis is used to infer some important aspects of the southwest Nova Scotia (Canada)‹Bay of Fundy (BoF) ecosystem with respect to the first year of life of Browns Bank (BB) haddock. One of the main observations in this ecosystem is that adult fish found in the BoF region consistently exhibit a larger length-at-age than fish found in the BB region. This is generally thought to be due to drift and retention processes acting during the early life stages, with the larger fish assumed to be the result of settlement in a more favourable temperature and food environment downstream in the BoF. Also, while BB is considered to be an important juvenile nursery ground and the source for the smaller southwest Nova (SWN) fish, the existence of the larger BoF fish suggests a downstream nursery ground. To date, no analysis has been made regarding the whereabouts of such a nursery ground. The assumption of a favourable temperature and food environment in the BoF is investigated. The result of this analysis suggests that the small Œinshore¹ region off of Yarmouth, Nova Scotia, is the best settling ground. An Age-1 capture probability map is constructed from 30 yr of Canadian research vessel (RV) survey data. This map shows 3 regions where juvenile haddock are found‹BB, the inshore, and upstream (northeast) of BB. The average pelagic juvenile concentration field derived from the biophysical model predicts that BB and the inshore region are preferred settling grounds. A habitat suitability map, based on surficial sediment type, shows that the observed settling grounds coincide with preferred haddock sediment type. Finally, it is shown that the upstream Age-1 fish most likely come from larvae that drifted downstream from the Western Bank haddock spawning ground, and that BB likely acts as a potential source for the Georges Bank haddock stock. Thus, the combination of biophysical modelling and analysis of fisheries data provides evidence of the metapopulation structure of haddock stocks on the Scotian Shelf/Gulf of Maine. KEY WORDS: Metapopulation · Haddock · Biophysical model · Scotian Shelf · Larval drift · Survival Full text in pdf format PreviousNextExport citation RSS - Facebook - Tweet - linkedIn Cited by Published in MEPS Vol. 263. Online publication date: November 28, 2003 Print ISSN: 0171-8630; Online ISSN: 1616-1599 Copyright © 2003 Inter-Research.

Microsatellite evaluation of haddock (Melanogrammus aeglefinus) stocks in the northwest Atlantic Ocean

The goal of this study was to gain insight about the impact of intensive fishing on a single haddock (Melanogrammus aeglefinus) stock, and examine the genetic structuring of spatially discrete spawning aggregations in the northwest Atlantic. We analyzed genetic change at four microsatellite loci for Georges Bank haddock over a 40-year time span in which significant changes in demographics and abundances have occurred in the population. Allelic diversities have changed little, indicating that, although the commercial fishery has collapsed, stock sizes have remained large enough to insulate against major reductions in genetic variation due to drift. Results indicate significant genetic divergence among decadally separated samples. Potential causes for these differences include admixture from other spawning regions, fluctuations in the effective number of spawners contributing to a single spawning event, drift, or a combination of these. Examination of discrete spawning aggregations from Georges Bank, Browns Bank, the Scotian Shelf, and Nantucket Shoals indicated significant differences among stocks. Genetic distance based measures supported the clustering of Scotian Shelf, Browns Bank, and Georges Bank haddock to the exclusion of Nantucket Shoals haddock. Haddock spawning on Nantucket Shoals may be genetically discrete from other haddock populations in the northwest Atlantic.

Stock Identification of HaddockMelanogrammus aeglefinuson Georges Bank Based on Otolith Shape Analysis

Abstract Otolith shape analysis was examined to determine its utility for stock identification of haddock Melanogrammus aeglefinus on Georges Bank. Otolith samples were collected from the Northeast Peak (eastern Georges Bank) and the Great South Channel (western Georges Bank) spawning components to examine stock continuity across Georges Bank. Otolith shape was described using 20 Fourier harmonics, four morphometric characteristics (area, length, width, and perimeter), and two shape indices (circularity and rectangularity). Potential confounding sources of variation (fish length, otolith position, age-group, and year-class) were examined and accounted for in the analyses before interpretation of spatial/stock differences. Significant age-specific differences in otolith shape between eastern and western Georges Bank haddock indicated stock separation across the Bank. Classification success for each spawning component ranged from 63 to 80% across age-groups and appeared to depend on growth rate differences....

Effects of First-time Spawners on Stock-recruitment Relationships for Two Groundfish Species

Recent experimental studies suggest that first-time spawning fish may not be as reproductively fit as repeat spawners. Traditional stock-recruitment models consider all mature fish as equivalent contributors to the spawning stock biomass. In this study we examine the effects of discounting first-time spawners on the stock-recruitment relationship of two species with varying life histories: Georges Bank haddock (Melanogrammus aeglefinus), a fast maturing gadoid, and Gulf of Maine witch flounder (Glyptocephalus cynoglossus), a slow maturing pleuronectid. Proportions of first-time spawning haddock ranged from 3 to 62 percent of the spawning stock biomass during 1963–96. Exclusion of all first-time spawners from spawning stock biomass improved the Ricker stock-recruitment relationship by 39 percent. For witch flounder, proportions of first-time spawners were less variable, never exceeding about 30 percent. Adjusting the spawning stock biomass for first-time spawners did not improved the overall relationship for witch flounder.

Effect of nonlinear predation rates on rebuilding the Georges Bank haddock (Melanogrammus aeglefinus) stock

The collapse of several northwest Atlantic groundfish stocks, including Georges Bank haddock (Melanogrammus aeglefinus), necessitated new fishing regulations and generated interest in the role of predation on stock productivity. The sharp break between prolonged periods of high (pre-1965) and low (post-1965) haddock abundance suggests differing levels of stock productivity, consistent with a surplus production model incorporating a nonlinear predation rate (Steele and Henderson's (1984. Science (Washington, D.C.), 224: 985-987) model). This model and a Schaefer (1957. Inter-Am. Trop. Tuna Comm. Bull. 2: 245-285) model without a predation term were fit to haddock data to evaluate various rebuilding strategies with two performance measures: the sums of discounted yield and discounted revenue. Steele and Henderson's model provided plausible parameter estimates for the entire data set (1931-1993) whereas Schaefer's model provided plausible estimates only for years of low productivity (1976-1993). The presence of multiple equilibria in Steele and Henderson's model resulted in minor shifts of F, potentially producing large shifts in projected future biomass. For either model, levels of F that maximize either yield or revenue were lower than the recently adopted target level of F0.1 = 0.24. Recent low production provides impetus for managers to consider a variety of plausible stock production models, and the uncertainty of production dynamics, in choosing rebuilding strategies.

A simple predator–prey model of exploited marine fish populations incorporating alternative prey

A simple two-species population model in which the predator is partially coupled to the prey is developed. The model is an extension of traditional two-species models but less complex than a three-species system. The growth rate of the predator depends upon predation on the modeled and alternate prey; this formulation provides greater realism in describing marine piscivores, such as spiny dogfish (Squalus acanthias), than two-species predator‐prey models. Two stable equilibria separated by a saddle point potentially exist for the predator‐prey system, and stochastic variability can lead to movement between equilibrium abundance levels. In addition, endogenous limit cycles may exist in the presence of predator fishing mortality. The model is applied to the predation of spiny dogfish on a representative groundfish species, the Georges Bank haddock (Melanogrammus aeglefinus). Stochastic variability is input to the model in the form of ‘‘red noise’’ (variance is a decreasing function of frequency), a feature observed in marine environments. Predator abundance can increase when the modeled prey abundance is low, due to consumption of alternate prey, consistent with the pattern observed in the spiny dogfish‐haddock abundances. Increased harvesting on the predator species allows the prey species to spend a greater proportion of time at the high equilibrium. The model presented here poses the interesting management problem of finding the optimal combination of fishing mortality rates for the two species. ? 1996 International Council for the Exploration of the Sea