Allee effects and the Allee-effect zone in northwest Atlantic cod

According to the theory of compensatory dynamics, depleted populations should recover when the threat responsible for their decline is removed because per capita population growth is assumed to be highest when populations are at their smallest viable sizes. Yet, many seriously depleted fish populations have failed to recover despite threat mitigation. Atlantic cod (Gadus morhua) stocks off Newfoundland, despite 30 years of dramatically reduced fishing mortality and numerous fishery closures, have not recovered, suggesting that drivers other than fishing can regulate the growth of collapsed fish populations, inhibiting or preventing their recovery. Here, using Bayesian inference, we show strong evidence of Allee effects in a south Newfoundland cod population, based on data on recruitment and spawning stock biomass. We infer the Allee-effect threshold, below which recovery is impaired. We demonstrate the necessity of data at low population sizes to make inferences about the nature of low-abundance dynamics. Our work indicates that Allee effects are not negligible in commercially exploited fish populations, as commonly projected, and that they represent an inhibitory force that can effectively prevent recovery from overfishing. Our findings contrast with prevailing fisheries management practices that assume compensatory dynamics at low abundances with potential to seriously overestimate the recovery potential of collapsed populations.publishedVersio

Effects of fishing during the spawning period: implications for management

Avoidance of fishing during the spawning season has been proposed as a contribution to achieving sustainable exploitation. Here we review the biological effects of fishing during the spawning period and explore their implication on sustainable management. A distinction will be made between direct mortality and indirect effects. The latter will review how fishery disturbance will affect the physiology and behaviour. Based on the results, a classification scheme is presented of the vulnerability for fisheries during the spawning period. Finally the implications for the population dynamics and fisheries management are discussed

Effecten van de visserij gedurende het voortplantingsseizoen op het duurzaam beheer van visbestanden: een uitgebreide Nederlandse samenvatting

In opdracht van Stichting Vis & Seizoen heeft IMARES een uitgebreide literatuurstudie uitgevoerd naar de mogelijke effecten van de visserij tijdens het voortplantingsseizoen. Dit rapport geeft een uitgebreide Nederlandse samenvatting van het Engelstalige rapport “Effects of fisheries during the spawning season: implications for management”

Measuring Fertilization in Populations of Sea Scallop (Placopecten magellanicus): Developing and Testing Methods in the Laboratory and Field

Most marine organisms are broadcast spawners, releasing their sperm and eggs into the water column. Methods of measuring in situ fertilization have proven successful with a few model species, which are reviewed in my introductory chapter. However, many commercially exploited species, such as the sea scallop Placopecten magellanicus, have been neglected. Sea scallop populations have greatly increased from fishing closures, but the mechanism behind this response is uncertain, particularly in regard to fertilization. In this dissertation I developed a methodology of measuring fertilization success and spawning events of P. magellanicus, tested it in laboratory and field settings, and developed a novel genetic probe to detect and quantify scallop gametes. Chapter 2 describes laboratory experiments and field results from our development of nylon mesh chambers used to measure fertilization success (percent of eggs fertilized) in situ. In dilution-series experiments, maximum fertilization success occurred at sperm concentrations \u3e107 sperm ml–1 . Between 8 and 24 h at ambient temperature, egg viability fell to zero. Sperm half-life shortened from 2 h to 9 min when sperm concentrations diluted by 10-fold from 107 cells ml–1 to 106 cells ml–1 . Flume trials demonstrated chamber artifacts: fertilization was lower inside the chamber than outside, and the effect was greater at higher flow rates, but chamber orientation to flow had no effect on fertilization. Increasing the numbers of eggs tended to reduce fertilization success. In dockside tests, a 30-fold difference in spawner numbers had a significant effect on fertilization success. In Chapter 3, I analyzed video surveys of scallop aggregations on western Atlantic fishery grounds to determine whether population density, degree of aggregation, and shell size were correlated with fishing closures. Based on these data, I created experimental benthic populations to measure fertilization success in situ. Fertilization success in these experiments did not vary significantly across a 10-fold difference in population density, a result which was inconsistent with the outcome predicted by a current fertilization model. This likely underscores the extreme variability in fertilization success in the field that is not captured by models. In Chapter 4 I developed and tested a genetic probe (Pmag_304F) and primer set (Pmag_282F, Pmag_492R) to detect and quantify P. magellanicus gametes in the water column. I used a TaqMan fluorescent probe and primer set to target the intergenic spacer region (ITS) in the scallop genome. To verify this probe works on scallop gametes, I tested it on replicate sperm dilution series. This method may be applied to field samples to detect and quantify spawning events for this species and other important invertebrates. This dissertation presents empirical data on the relationship between spawner abundance and fertilization success in P. magellanicus, evidence for a possible component Allee effect, some form of compensation at low densities and the development of two methods to detect spawning events in the field. These new tools and data improve our understanding of a previously poorly studied aspect of scallop reproduction, and may provide insight into their resilience to fishing pressure

Five centuries of cod catches in Eastern Canada

The fishery for Northern Atlantic cod (Gadus morhua) off Newfoundland and Labrador, Eastern Canada, presents the most spectacular case of an exploited stock crashed in a few decades by an industrial bottom trawl fishery under a seemingly sophisticated management regime after half a millennium of sustainable fishing. The fishery, which had generated annual catches of 100000 to 200000 tonnes from the beginning of the 16th century to the 1950s, peaked in 1968 at 810000 tonnes, followed by a devastating collapse and closure 24 years later. Since then, stock recovery may have been hindered by premature openings, with vessels targeting the remains of the cod population. Previous research paid little attention towards using multicentury time series to inform sustainable catches and recovery plans. Here, we show that a simple stock assessment model can be used to model the cod population trajectory for the entire period from 1508 to 2019 for which catch estimates are available. The model suggests that if fishing effort and mortality had been stabilized in the 1980s, precautionary annual yields of about 200000 tonnes could have been sustained. Our analysis demonstrates the value of incorporating prior knowledge to counteract shifting baseline effects on reference points and contemporary perceptions of historical stock status.publishedVersio

Tensions in the communication of science advice on fish and fisheries: Northern cod, species at risk, sustainable seafood

Providing science-based advice can be challenging. Personal in its reflections, the story that follows asks throughout: What constitutes an appropriate model for the communication of science-based advice that best serves society? The first “front line,” in 1992, involved tenuous hypotheses on the collapse and recovery of Newfoundland's Northern cod (Gadus morhua), raising troubling questions about political influence on science-based advice and on its integrity. These questions subsequently motivated a critique written with two colleagues on the communication of science to decision-makers, provoking a telling invective from a government department in defence of the status quo. The story transitions to my 2000–2012 tenure as a member and then as chair of Canada's national body advising which species should be on the legally binding national at-risk register, illustrating how politically sensitive science-based advice can be objectively, effectively, and independently communicated, unfiltered by vested interests. Since 2009, I have served as independent science advisor on the sourcing of sustainable seafood to Canada's largest food retailer, providing a meaningful, impactful opportunity to advise their decision-makers. Science-based advice, free from political and advocacy-driven vested interests, is a requisite return for tax-supported investments in science. If provision of such advice is a “moral imperative,” as argued more than 60 years ago by C.P. Snow, then scientists are obliged to be the best advisors that we can be.publishedVersio

Tensions in the communication of science advice on fish and fisheries: Northern cod, species at risk, sustainable seafood

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Influence of the northeastern Atlantic oceano-meteorological variability on the northern hake ("merluccius merluccius"). Analysis of the 1978-2006 period

316 p. : il., graf., map.[EUS]Tesi honetan iparraldeko legatz europarraren populazio-dinamika ikertu eta inguruneko parametroen eragina aztertu da 1978-2006 artean. Gainera, bizitzako lehen faseetan legatzaren kalteberatasunaren atzean dauden kausak zeintzuk diren ere identifikatu dira. Komertzialki hain espezie garrantzitsua izanik, arrantzaren eragina nabaria da legatzaren biomasak 80ko hamarkadan jasandako jeitsiera zorrotzean. Legatz europarraren denboran zeharreko erreklutamenduak dentsitatearekiko menpekotasuna erakutsi du, gurasoen biomasak urteroko erreklutamenduan duen garrantzia azpimarratuz. Hala ere, ezin da ziurtatu stock-reklutamendu erlazio horrek Ricker itxura hartzen duenik, datu falta baitago, batik bat, stock-aren balio baxuetarako. Bestalde, inguruko aldakortasun ozeano-klimatikoak zeresan handia dauka legatzaren erreklutamenduaren arrakastan. Ikusi da legatzaren bizitzako lehen faseetan tenperatura epelagoa izatea mesedegarri izan daitekeela legatzarentzat (bai kondizio egokien eremua/sasoia zabaltzen delako, edota hazkuntza-tasa altuagoengatik). Gainera, kostaranzko garraioak fase sentikorren dispertsioa saihestuko luke, eta, errute-eremuetatik hazkuntza-eremuetarako garraioa lagunduko luke. Baldintza egoki hauek garrantzi handikoak izan dira legatzaren erreklutamenduaren arrakasta ziurtatzeko, batik bat, legatzaren biomasa balio izugarri baxuetan aurkitu zen kasuan. Amaitzeko, ikerketa honen emaitzek iradokitzen dute erregimen-aldaketa ekologiko bat gertatu zela Atlantikoko ipar-ekialdeko plataforma kontinentalean 1988/89 urteetan. Erregimen-aldaketa hau eskala globalean ez ezezik (NAO, Golkoko korronte indizea eta Ipar Hemisferioko tenperaturen anomaliak), eskualde-mailan ere gertatu zen (Atlantikoko ipar-ekialdeko klimatologia eta Zelta itsasoko kopepodoen aldakortasuna). Legatzaren erreklutamendu-arrakastan gertatutako aldibereko aldaketa kontuan izanik, ondorioztatu da sasoi horretako ingurunea legatzarentzat onuragarria dela.[ES]En esta tesis se ha estudiado la dinámica poblacional del stock del norte de la merluza europea, así como la influencia de los parámetros ambientales durante el periodo 1978-2006. Además, se han identificado las variables que afectan la vulnerabilidad de lo estadios tempranos de la merluza. En una especie comercial como la merluza, la presión pesquera ha hecho que la biomasa de esta especie haya disminuido significativamente en la década de los 80. El reclutamiento de la merluza europea ha mostrado ser dependiente a la densidad del stock, y, esto subraya, la importancia que la biomasa parental tiene en el reclutamiento anual. Sin embargo, no se puede demostrar que esa relación stock-reclutamiento siga el modelo Ricker, debido a la falta de datos, sobre todo para valores bajos de biomasa. Por otra parte, la variabilidad océano-climática tiene mucha importancia en el éxito del reclutamiento de la merluza. Se ha demostrado que en los primeros estadios de vida de la merluza, una temperatura más cálida puede ser favorable para esta especie (se extiende el área/periodo óptimo, o/y las tasas de crecimiento son mayores). Asimismo, el transporte hacia la costa evitaría la dispersión de huevos y larvas, y favorecería el transporte de estas fases vulnerables desde las áreas de desove hasta las zonas de crecimiento. Estas condiciones favorables son de gran importancia a la hora de asegurar el éxito en el reclutamiento de la merluza, sobre todo cuando la biomasa de la merluza se encuentra en valores extremadamente bajos. Finalmente, los resultados de este trabajo sugieren un cambio de régimen ecológico en la plataforma continental del Atlántico noreste en los años 1988/89. Este cambio de régimen se detectó no sólo a escala global (NAO, el índice de la corriente del Golfo y anomalías de temperatura en el Hemisferio Norte), sino también a nivel regional (climatología del Atlántico noreste y variabilidad en la abundancia de copépodos en el mar Celta). Considerando que el cambio en el éxito del reclutamiento tuvo lugar en sincronía con el cambio ecológico, se concluye que el entorno de la merluza se volvió favorable para su supervivencia.The work carried out in this Dr. Philos Thesis has been developed in AZTI-Tecnalia and it has been financed by: Fundación Centros Tecnológicos Iñaki-Goenaga-k emandako “Oceanografía y recursos marinos” doktoretza-aurreko beka 2005-2009

A decade of monitoring Atlantic cod Gadus morhua spawning aggregations in Massachusetts Bay using passive acoustics

© The Author(s), 2020. This article is distributed under the terms of the Creative Commons Attribution License. The definitive version was published in Caiger, P. E., Dean, M. J., DeAngelis, A. I., Hatch, L. T., Rice, A. N., Stanley, J. A., Tholke, C., Zemeckis, D. R., & Van Parijs, S. M. A decade of monitoring Atlantic cod Gadus morhua spawning aggregations in Massachusetts Bay using passive acoustics. Marine Ecology Progress Series, 635, (2020): 89-103, doi:10.3354/meps13219.Atlantic cod Gadus morhua populations in the northeast USA have failed to recover since major declines in the 1970s and 1990s. To rebuild these stocks, managers need reliable information on spawning dynamics in order to design and implement control measures; discovering cost-effective and non-invasive monitoring techniques is also favorable. Atlantic cod form dense, site-fidelic spawning aggregations during which they vocalize, permitting acoustic detection of their presence at such times. The objective of this study was to detect spawning activity of Atlantic cod using multiple fixed-station passive acoustic recorders to sample across Massachusetts Bay during the winter spawning period. A generalized linear modeling approach was used to investigate spatio-temporal trends of cod vocalizing over 10 consecutive winter spawning seasons (2007-2016), the longest such timeline of any passive acoustic monitoring of a fish species. The vocal activity of Atlantic cod was associated with diel, lunar, and seasonal cycles, with a higher probability of occurrence at night, during the full moon, and near the end of November. Following 2009 and 2010, there was a general decline in acoustic activity. Furthermore, the northwest corner of Stellwagen Bank was identified as an important spawning location. This project demonstrated the utility of passive acoustic monitoring in determining the presence of an acoustically active fish species, and provides valuable data for informing the management of this commercially, culturally, and ecologically important species.Thanks to Eli Bonnell, Genevieve Davis, Julianne Bonell, Samara Haver, and Eric Matzen for assistance in MARU deployments, Dana Gerlach and Heather Heenehan for help in passive acoustic data analysis, and the NEFSC passive acoustics group for useful discussions. Funding for 2007−2012 passive acoustic surveys was provided by Excelerate Energy and Neptune LNG to Cornell University. Fieldwork for 2013−2015 was funded through the 2013−2014 NOAA Saltonstall-Kennedy grant program (Award No. NA14NMF4270027), and jointly funded by The Nature Conservancy, Massachusetts Division of Marine Fisheries, and the Cabot Family Charitable Foundation. Funding for 2016 SoundTrap data was provided by NOAA’s Ocean Acoustics Program (4 Sanctuaries Project)