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Cod Research in Massachusetts and the Release of a New StoryMap

By the time Pilgrims arrived in Plymouth, Massachusetts in 1620, the most prominent feature along the coast had already been known as Cape Cod for a generation. Cod fishing was North America’s first industry and is still a central part of our heritage. Once an emblem of pride and prosperity, the cod fishery of today is drastically different, challenged by declining populations, warming ocean temperatures, and lost spawning grounds. No one group has all the answers for fixing these issues, but the Massachusetts Division of Marine Fisheries contributes extensive research to help improve stock assessments and fishery management.

Because of the multispecies nature of this fishery, a low cod quota acts as a “choke stock” for most fishermen, preventing them from accessing their available catch allocation of more abundant stocks. In other words, when fishermen use up their cod allocations, they are faced with an unfortunate choice: either cease fishing for the year or lease quota from others at exorbitant rates that often exceed the market price of the fish. For this reason, much attention has been focused on the credibility of the stock assessments that catch limits are based on. Many fishermen find it hard to believe that such extreme conservation measures are necessary.

Industry Based Survey

To help understand these different perspectives on the status of the cod stock, DMF initiated a new bottom trawl survey to address many of the questions underlying the fishing industry’s disbelief in stock assessments. Through several meetings with fishermen and scientists, we developed a survey approach to intensively sample the core area for Gulf of Maine cod using standardized equipment and practices aboard commercial fishing vessels, but according to a randomized design that provides scientific credibility.

This effort, known as the Industry-Based Survey (or IBS), completed over 3,500 standardized tows from Chatham, MA to the Canadian border and collected biological data, such as length, age, sex, and maturity from over 100,000 individual cod. Both the IBS and the stock assessment agree that Gulf of Maine cod have declined dramatically in recent years. However, the survey disagreed with the assessment on age distribution of the population, which may be contributing to consistently poor management outcomes. The datasets generated by the IBS are being used in various ways to improve how we assess and manage cod in the Gulf of Maine.

Protecting Spawners

Understanding the occurrence and persistence of spawning grounds is critical to the protection of cod stocks. Atlantic cod reproduce in dense aggregations that form at specific locations and seasons. This predictability and abundance make spawning cod an easy and lucrative fishing target, but also vulnerable to disruption. A complex sequence of behaviors takes place prior to mating and intense fishing pressure can cause cod to abandon a spawning ground, essentially canceling reproduction at that site for the year. Many spawning grounds in the Gulf of Maine have been lost entirely. The future of the cod stock, and the fishery that it supports, relies on the continued existence of the remaining spawning aggregations – we need to let these fish complete their life cycle.

In addition to interviewing knowledgeable fishing captains, DMF scientists use high-tech tools like acoustic telemetry, autonomous underwater vehicles, and marine sound recorders to identify and map cod spawning grounds. In collaboration with commercial fishermen, NOAA, the Nature Conservancy, and the University of Massachusetts, we have now described all the cod spawning grounds known to remain in the Gulf of Maine and on Georges Bank. From this work, closed areas and seasons have been created that allow spawning cod the time and space they need, while minimizing the impact to our fisheries.

Improving Assessments

Wild fish populations are constantly subject to change. A primary goal of fishery management is to respond to these changes, regulating harvest to ensure the sustainability of the resource. To do this, we must accurately measure population change over time. This is the fundamental purpose of a stock assessment – to act as a mathematical ‘model’ of the fish population that can inform harvest regulations.

Age-based stock assessment models are the best available tool for setting catch limits. Although these state-of-the-art models incorporate a wide variety of data from the fishery and scientific surveys, there are many assumptions that must be made. It is critical to get these assumptions as correct as possible, or else the resulting harvest regulations will cause undue harm to the fish population, the fishery, or both.

A critical first step when assessing a “stock” of fish is to define its boundaries. The goal is to identify the limits of where the fish population completes its life cycle. For the past 40 years, cod in US waters have been separated into two stocks: Gulf of Maine and Georges Bank. A recent in-depth review of cod stock structure is re-drawing this map, which now includes five genetically-distinct stocks. MADMF research was integral to this effort.

One of the most difficult challenges confronting stock assessment and fishery management is the presence of two overlapping cod stocks in the Western Gulf of Maine. These two groups of fish maintain reproductive isolation by spawning at opposite times of year, yet intermix when not spawning. MADMF scientists developed a method to identify which group these cod belong to by examining calcified structures known as otoliths found inside fish skulls.

Not all sizes of cod are vulnerable to capture by the fishery. For example, smaller fish can escape through the large mesh netting used for gillnets and trawls. Alternatively, large adult cod become less vulnerable to capture when spawning grounds are closed to fishing. This size or age-based vulnerability to capture is known as “fishery selectivity” and is a key variable in a stock assessment model. To make the math simpler, assumptions are often made about the shape of the selectivity “curve” and how it varies over time. Using the Industry-Based Survey dataset, MADMF scientists have independently estimated the size selectivity of the Gulf of Maine cod fishery, demonstrating that assumptions used in the past need to be updated.

One way to check the assumptions of a stock assessment is to compare the results to independent estimates of population size. This is typically done by extrapolating the catch rates observed in a trawl survey to the entire stock area. However, for these population estimates to be credible, the behavior of fish when confronted by the survey trawl needs to be accounted for. Using carefully-designed experiments, sidescan sonar, and machine learning, we estimated the “herding effect” of the IBS trawl net, as well as the escapement of small fish beneath its footrope. These measures of trawl efficiency have allowed DMF scientists to provide realistic estimates of the size, trend, and age distribution of the Gulf of Maine cod stock. Although the assessment and Industry-Based Survey agree that the stock has declined dramatically in recent decades, they differ on the population age distribution, which may be related to the assessment assumptions about fishery selectivity.

Reducing Cod Bycatch and Discard Mortality

Over the past 5 years, more than half of the cod removed from the Gulf of Maine came from recreational angling. Most of these fish were never actually brought home for dinner – instead, they died from capture-related injuries after being released.  Due to strict limits on the numbers, size, and seasons for recreationally caught cod, there are many reasons why cod must be discarded. Understanding how many of these fish survive after being released is critical to fishery management and stock assessment. Working with our scientific partners (the New England Aquarium, University of New England), DMF has developed state-of-the-art methods to estimate discard mortality and to identify the factors influencing survival.

One clear way to reduce the number of cod that die after release is to avoid catching them in the first place. This sounds much easier to do than it is. Cod and haddock (a close relative) share similar habitats and are often caught together by the fishery. As cod fishing rules have tightened with the declining population, unprecedented growth in the haddock stock has attracted recreational anglers back out to sea. Discarded bycatch of cod by haddock anglers is now a leading source of mortality for the cod stock.

Using the Industry-Based Survey dataset, DMF has identified times and areas with excellent haddock fishing, but little cod bycatch. This information has been summarized in an easy-to-understand Recreational Haddock Guide that is available online, in print, and via a location-aware smartphone app. Before being released to the public, the  Haddock Guide  maps were rigorously tested by charter boat captains who reported their catch rates of cod and haddock from over 800 locations. A fleet of citizen scientists continue to test the Haddock Guide maps each year, making sure they are still relevant.

Understanding Perspectives

Although the Industry-based Survey and stock assessment agree that Gulf of Maine cod have declined by 80% over the past decade, most fishermen believe the stock has increased. How can this be? DMF scientists are trying to answer this question by examining the relationship between population size, regulations, catch rates, and fishery perceptions.

As the stock declined and access to fishing grounds (“days-at-sea”) were cut, the amount of cod that could be landed each day was increased several times to reduce the wasteful discarding of fish. Understandably, this led fishermen to target cod more, which caused catch rates to increase while the stock declined. Although a switch to annual catch limits in 2010 has now aligned the incentive to target cod with the population size, the influence of past management choices is still impacting fishery perceptions today.

Not all sizes of cod have declined at the same rate. Recent poor-year classes mean less juvenile cod and a high mortality rate means fewer large cod than in previous decades. The lack of juvenile cod goes unnoticed by fishermen because they are required to use large mesh nets and hooks that only catch adults. Additionally, fishermen are unable to witness the loss of the largest cod because they are prohibited from accessing spawning grounds and other prime habitat. The medium-sized cod (“scrod” and “markets”) most frequently caught by the fishery have declined the least, particularly along the Massachusetts coast. A decrease in the minimum fish size in 2013 has allowed the fishery to land more of these fish, adding to the perception of an increase.

Stakeholders and policy makers should recognize this unavoidable influence that regulations have on fishery perceptions. We are hopeful that by offering a translation between the scientific and and fishery perspectives, each group can acknowledge the validity that underlies the other, and future conflict can be avoided.

The Future of Cod

The research conducted by DMF and our partners has significantly advanced the understanding of cod population dynamics. From creating spawning closures to re-defining stock boundaries to accounting for discard mortality, our findings are already being applied at multiple levels to improve fishery management and stock assessments. We are hopeful that these changes will help alter the trajectory of the cod stock and ultimately improve the conditions for our struggling fishery. However, many important questions remain to be answered and we will continue to use our expertise to identify and fill these critical knowledge gaps.


A new online StoryMap was recently created to showcase the various cod-focused research projects that have been completed by DMF over the past decade. The StoryMap offers an immersive multimedia experience that includes photos, videos and sounds collected from both above and below the water’s surface. Follow along as we use electronic tracking, underwater drones, sidescan sonar, machine learning, and citizen science to reveal the fascinating lives of Atlantic cod. The StoryMap can be found here.

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