Shifting environmental conditions in the Northwest Atlantic Ocean has contributed to the rise and decline in fish populations over the past 75 years, said a new scientific research paper.
Marine Institute LinkedIn photo.
Environmental control on the productivity of a heavily fished ecosystem, authored by Memorial University Marine Institute researcher Dr. Frédéric Cyr and seven co-authors with Fisheries and Oceans Canada, was published June 6 in the journal Nature Communications.
Using decades of climate and fisheries data dating back to 1950, the researchers found that the Northwest Atlantic experiences shifting climate phases that can range from three years to 20 years.
“The ocean climate of the Northwest Atlantic changes in cycles and these cycles impact the fish stocks. The frequency of these cycles seems to have increased in recent decades, which may have slowed down the recovery of some groundfish stocks after the 1990s collapse,” said Dr. Cyr, research scientist with the Marine Institute’s Centre for Fisheries Ecosystems Research in a media release.
“Tracking these climate shifts offers a powerful tool for risk assessment and adaptability in fisheries management, for example, by reducing the fishing pressure when the climate is negatively affecting the stocks.”
The climate phases, which can be classified as warm and cold, influence both ecosystem productivity and fisheries of groundfish species. Ecosystem productivity refers to the rate at which species biomass is generated in an ecosystem.
The paper said the warm and cold phases also impact phytoplankton and zooplankton that fish populations rely upon for food. During warmer phases in the ocean ecosystem off Newfoundland and Labrador, for instance, the density of phytoplankton and zooplankton was higher than during colder phases.
While overfishing was a significant factor in the collapse of Northern cod, the paper said environmental factors have also contributed to the collapse and may explain the lack of recovery more than 30 years later.
“The collapse of groundfish stocks occurred during one of the coldest periods of the last century in the Northwest Atlantic and coincided with the collapse of capelin, a key pelagic forage fish species for N.L. ecosystems, and several groundfish species that were not subject to directed commercial fishing. Similar cold conditions occurred in the mid-2010s, which did not help the recovery,” said Dr. Cyr.
“In this work, we argue that the lack of precise quantitative knowledge on the effect of the environment on fish stocks does not preclude its integration into stock assessments and its consideration in fisheries management decisions. Using more than 70 years of data from the N.L. shelves, we show that the ecosystem goes through productivity phases on decadal time scales. We also show that these phases can be identified using proper environmental monitoring and be used to guide fisheries management. For example, more conservative fisheries management policies (e.g., lower extraction rates, stronger emphasis on conservation when evaluating the trade-off between stock conservation and socio-economic objectives) could be used in phases of low productivity, and more relaxed fisheries management policies (e.g., higher extraction rates, increased emphasis on socio-economic objectives) could be used in phases of higher productivity,” read the study.
Eight climate phases, generally characterized by warmer or colder ocean conditions, were identified between 1951 and 2017 in the study.
“A ninth warm and potentially more productive climate phase has emerged since 2018. Although its effect on fish stocks has yet to be quantified, some signs of improvement in the finfish community have already been detected. These phases are driven by large-scale atmospheric conditions…. The warmer phase emerging since about 2018 may, however, signify a transition to a more productive phase in the coming years, consistent with a recent improvement of the capelin fall condition index.”
The study chronicles seven decades of important milestones for the N.L. fisheries that have been linked to the ocean climate.
“Following centuries of relatively small-scale fisheries, the post-World War II period saw the introduction of large, mechanized fishing vessels, including factory trawlers. Historical reconstructions suggest that cod catches remained below 300,000 tonnes/year for hundreds of years before rapidly increasing in the 1950s and 1960s, reaching a peak of more than 800,000 tonnes in 1968. Although the capabilities of this new fishing fleet were undeniable, it should be noted that record catches of the 1960s were achieved during one of the warmest periods in at least seven decades on the N.L. shelves. This period was characterized as uniquely favorable for groundfish productivity.
“A first partial collapse of the groundfish fisheries occurred in the early 1970s during a relatively cold phase that was in sharp contrast to the warm climate of the 1950s and 1960s (note that 1972 is the third coldest year recorded by the Newfoundland and Labrador Climate Index [NLCI], tied with 1984). This collapse on the N.L. shelves coincided with general decreases in Atlantic cod stocks across the NW Atlantic and was attributed to poor environmental conditions. A partial recovery followed in the late 1970s, a period again characterized by warmer ocean conditions and positive sea level pressure (SLP) anomalies above the pole. This partial recovery of groundfish stocks was also aided by a reduction in fishing pressure in coastal waters following international agreements that led to the establishment of exclusive economic zones of 200 miles for maritime nations.
“From the early 1980s to the late 1990s, the NW Atlantic entered its coldest phase in the last 70 years (a first cold pulse was recorded in the mid-1980s and a second in the early 1990s). Multiple commercial and non-commercial fish populations successively collapsed during this period. The biomass of the capelin stock in Northwest Atlantic Fisheries Organization (NAFO) Divisions 2J3KL rapidly collapsed from 5,800 kt in spring 1990 to 600 kt in fall 1990 — probably due to environmental drivers rather than fishing — before the collapse of the Northern cod stock. The collapse of cod led to the eventual establishment of a series of moratoria on groundfish fisheries beginning in 1992, while 1991 and 1993 were the coldest years recorded by the NLCI. In the following decades, some coldwater shellfish stocks became more productive, although their increases in biomass never fully compensated for losses in groundfish biomass. Although some finfish stocks have shown signs of improvement in the following decades, the recovery of many groundfish and pelagic stocks appears to have stalled during the 2014–2017 phase. This period again coincides with a shift in climate towards colder conditions, with the 2014–2017 phase sharing some similarities with that of the early 1990s, albeit of much shorter duration. Following a reanalysis of cod stocks using a longer time series, the Northern cod commercial fishing moratorium ended in 2024.”
“We also note that in addition to 2014–2017, another cold phase of lesser magnitude (2006–2009) interrupted the otherwise positive run of the NLCI that occurred between the late 1990s and the mid-2010s. Since the late 1990s, the N.L. climate has exhibited alternating colder and warmer conditions over relatively short phases, and the NLCI has not returned to a positive run similar to the one observed in the 1950s and 1960s. Provided that the ocean climate is influential on fish stocks, this means that the climatic conditions and the setup preceding the record catches of the 1960s have not been replicated over the past 50 years.”
The study authors “propose an approach to identify when an ecosystem is characterized by particular productivity regimes and argue that it offers a mechanism for adjusting fisheries management given changes in climatic conditions going forward… Linking environmental fluctuations with fish stock dynamics is a key step in the implementation of EBFM (Ecosystem-based fishery management). Towards this goal, this study demonstrates how 70 years of fluctuations in N.L. ecosystems productivity and changes in fish biomass can be explained using a simple climate index. Recognizing the phases (or productivity regimes) that an ecosystem is subject to, such as the ones described in this study, provides a powerful tool to inform fisheries management. This environmental information can be taken into account either qualitatively or quantitatively… Fisheries management actions in relation to a changing climate are plagued by many unknowns that should nevertheless be considered when setting management objectives, since these decisions can affect the future state of natural resources. This work joins a growing body of literature that proposes pragmatic solutions to how to overcome these challenges.”
