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Atlantic Herring
 Herring Research: Assessing Population Structure in Gulf of Maine Herring

Overview | Stocks | Stock Differentiation | Management and Computer Models | References

Much of the Atlantic herring research on both sides of the North Atlantic is directed at understanding the dynamics of the different herring stocks, including migration patterns, intermixing rates, spawning site fidelity and spawning stock integrity, fishing mortality rates and stock structure. Researching the population dynamics of the migratory, pelagic Atlantic herring (with spawning a short-term event that takes place in murky water, and populations intermixing offshore) is a difficult task. Tupper et al. point out that the intermixing behavior makes it difficult for fisheries biologists to accurately estimate the population dynamics of Atlantic herring.

Generally, fisheries managers have considered that there are distinct, manageable stocks in the northwest Atlantic. There is considerable evidence in favor of distinct stocks, including homing, larval retention and predictable spawning sites/times. For example, it has been demonstrated that approximately 90% of herring in Newfoundland waters return to the same area to spawn in successive years. [1]

The extent to which herring stocks intermix is a long-standing question for herring biologists and a problem that is far from solved, however. When different stocks occupy the same feeding grounds, for example, do they maintain their discrete identities? How closely are the different stocks related genetically? When the fish leave the feeding grounds, do the individual fish faithfully segregate into their separate spawning stocks? These kinds of questions have not yet been fully answered, and have important implications for fisheries management. The problem is particularly perplexing because, despite years of research and data, the results are inconsistent. Distinct spawning areas and evidence of homing and larval retention suggest segregated, distinct populations, while morphological, tagging and genetic studies have not fully supported this concept.

While some researchers are searching for answers in the wild using traditional techniques, tagging and measuring adult fish, locating egg beds and tracking herring larvae, other researchers are using genetic techniques and computer models in the laboratory to tackle the questions from entirely new angles.

 

Gulf of Maine Stocks: Who's Who in the Herring World

Just what is a stock of fish, anyway? A stock of fish is defined as a group of fish that remain sufficiently isolated from others so as to be manageable as a distinct unit.

Herring biologists generally agree that there are three major stocks or stock "composites" of herring in the Gulf of Maine and the Georges Banks area that return to the same spawning grounds every year. Each of these stocks may in turn be composed of smaller groups that spawn on individual banks or coastal locations. The three main Atlantic herring spawning stocks in the Gulf of Maine are:

  1. Southern Nova Scotia Stock: Trinity Ledge / Lurcher Shoals / German Bank.
  2. Gulf of Maine stock: Coastal waters from Grand Manan island south to the Isles of Shoals (New Hampshire), and on off-shore banks such as Jeffery's Ledge.
  3. Georges Bank stock: Georges Bank /Nantucket Shoals.

Herring stocks are found on both sides of the Gulf of St. Lawrence and along the southwestern and eastern coast of Newfoundland.

Overview | Stocks | Stock Differentiation | Management and Computer Models | References

 

How can we tell one stock from another?

Fish stocks can comprise distinct biological units, separated from other stocks of the same species by geographic isolation, genetic isolation, or different environmental (e.g. temperature, salinity) requirements. The differences between one stock and another can be extremely subtle and difficult to quantify. Herring researchers employ a number of different research tools to identify and study the dynamics of different herring populations. These different research techniques and approaches, described below, help piece together the story of who's who in herring.

Tagging and Sampling | Otoliths | Morphometrics and Meristics | Eggs and Larvae | Parasites | DNA

 
Tagging and Sampling
Machine tagging streamlines the process of inserting extremely small tags into herring for release and recapture. (Northwest Marine Technology and Shale Rosen photos)

Historically, one tried-and-true method for gathering data about a fish stock has been to catch and tag as many fish as possible, release them, then try to catch them again. Biologists refer to this data harvest as "tag returns." Fish tagging is a very important tool for fisheries scientists. Tag return data helps biologists analyze many characteristics of a fish population including distribution and migration patterns, population structure, and degree of stock isolation versus intermixing, to name a few. Currently, both the Maine Department of Marine Resources (DMR) and Canada's Department of Fisheries and Oceans (DFO) are conducting tagging studies of Atlantic herring in the Gulf of Maine.

Another basic research technique is to analyze samples of harvested fish, recording basic statistics such as length, weight, sex and maturity of each individual fish. From these data, a great deal of information can be garnered about the biological characteristics that distinguish herring belonging to different stocks and the degree to which they mix.


Otoliths

Knowledge of the age-structure of different fish populations is important for fisheries management. Because the growth of fish (including herring) is highly dependent on genetic and environmental (temperature, food availability) factors, length is not tightly coupled with age. A more accurate way to determine the age a fish is to remove and analyze its otoliths - the small, calcareous "ear bones" used by the fish for balance and orientation. Otoliths have been used for years to age fish. Otoliths have distinct daily and annual growth rings, not unlike the growth rings of a tree. These rings can be studied with the use of a microscope.

The otoliths sampled from one stock of fish vary in detectable ways from otoliths of a different stock. Analysis of subtle variation in otolith structure can be used to further define herring stocks.

See also the Northeast Fishery Science Center's Fishery Biology Program page on herring otoliths.


Morphometrics and Meristics

Detailed analysis of the morphology and anatomy of a fish can help researchers study different herring stocks. Morphometrics, the "measurement of morphology" or form of an organism, is a scientific method used to quantify shape differences among organisms. In its pure scientific application, morphometrics is an intricate process that measures the exact shape of a fish.

herring with morphometrics lines overlayed

Mathematical algorithms are used to analyze the measurement data collected at certain anatomical landmarks on the body of a fish. Measurements are painstakingly recorded and statistically analyzed to examine differences in form, contour, profile, shape, and the exact dimensions of organisms. Morphometrics is used by researchers to distinguish among different species of closely related fish, or among different stocks of the same species of fish. New research on herring suggests that morphometrics might play a valuable role in helping define herring stocks in the Gulf of Maine.

Meristic characters in an organism are characteristics that show repetition. Meristics, the discipline that studies variation in these repetitive characteristics, has shown some promise. The exact number of rays or spines in the pectoral fin or the number of stomach folds inside a herring are established when the herring is young and does not change over time. For example, researchers are able to count the number of scales along the lateral line of different fish, then compare the results from two different stocks in order to detect meristic variation between the two groups.

In the case of Atlantic herring, researchers have analyzed meristic variation using a variety of characteristics, including pelvic, anal, and pectoral fin ray counts, as well as gillraker counts and pyloric caecae (protrusions or folds on stomach wall) counts. Because meristic characters are environmentally and genetically determined, they can be used as indicators of genetic variation or stock structure as well as the effects of environmental changes over time.

As another example, pectoral fin ray counts were conducted in the early 1960s for juvenile herring found in Maine coastal waters. [2] The results of this meristic analysis suggested that the fish originated from a Georges Bank spawning stock. In less than a decade, the fin-ray counts had changed and continued to change for subsequent generations of coastal Maine herring. Researchers argued that decreasing seawater temperatures in the Gulf of Maine region during this time period may have altered the herring's environment enough to cause detectable variation in the number of pelvic fin rays.


Eggs and Larvae - Early Life Stages and Stocks

Information about herring stock dynamics can be gained from studying the distribution of herring spawning grounds and larvae. A recent project by the Island Institute on herring spawning site distribution is described in detail under this site's section, spawning research. Homing (returning to the same spawning grounds) and larval retention (larvae held in a certain area by current/tidal patterns) can provide evidence in support of distinct, localized spawning populations.

Some researchers have argued that the size of tidally mixed areas (which may retain larvae) determines the size of the local herring stock. Others argue that currents transport significant numbers of larvae away from spawning grounds. Many research projects in the 1980s and early 1990s attempted to relate larval distribution and dispersal to spawning stock structure in the Gulf of Maine. These studies are reviewed in Tupper et al.

Researchers at Canada's Dalhousie University are currently researching the effects that prespawning events (e.g. oceanographic and environmental conditions) have on how many young herring successfully recruit to (join) Scotia-Fundy herring populations. These results will be compared to those for Icelandic populations of herring. [3]


Parasites

Another tool available to herring biologists is analysis of the different parasites associated with different populations of fish. For example, the parasites of a herring stock in the Gulf of St. Lawrence may be different from parasites in a Georges Bank herring. One parasite, the "herring worm" (Anisakis simplex), is less common in the Bay of Fundy than other regions of Atlantic Canada. [4] Worms and other parasites found in herring can act as biological indicators for scientists looking to distinguish one stock from another.

 
DNA Analysis

Genetic testing and DNA fingerprinting are currently popular subjects in the media, and in the sciences genetic techniques are being applied across the board in a number of fields, from marine larval ecology to applied biomedical research. The field of herring research is no exception. Genetic analysis using herring DNA represents a powerful and relatively new method to address questions of stock structure and spawning site fidelity. Using just a fin clip or a plug of tissue from herring, a scientist can investigate how closely related the different stocks are genetically. The beauty of genetic techniques is that they can be applied equally well to all life stages - including the pelagic larval phase. The application of genetic research to larval research has great potential that has not yet been fully realized.

To date, genetic research has not provided strong evidence of distinct populations of Atlantic herring, and may indicate that the groups only recently became segregated. [5] The genetic approach may be most useful in distinguishing among stocks separated by relatively large distances (e.g., Georges Bank vs. Scotian Shelf vs. Gulf of Saint Lawrence). Genetic analyses provided evidence that the recent reappearance of herring on Georges Bank represents a resurgence of the historical population rather than colonization from another stock. [6]

Current research at Dalhousie University in Halifax is utilizing genetic techniques (with both adult and larval samples) to detect differentiation among herring populations in the Scotia-Fundy region and then analyze that differentiation in the context of oceanographic processes. Preliminary results indicate that herring populations from the Scotia-Fundy region are distinct from both the Pacific and northeast Atlantic herring. [7]

Researchers at the University of Maine at Orono are carrying out similar genetic research that is conducted at a smaller spatial scale. The University of Maine project is attempting to detect genetic differentiation among spawning populations in the Gulf of Maine (Jeffreys Ledge, Schoodic Ridge, Georges Bank, and Trinity Ledge). [8] These genetic studies may provide some answers to the questions of spawning site fidelity and intermixing rates of different herring stocks.

Tagging and Sampling | Otoliths | Morphometrics and Meristics | Eggs and Larvae | Parasites | DNA
Overview | Stocks | Stock Differentiation | Management and Computer Models | References

 

Herring Management and Computer Models

In recent years, computer models have been applied to a variety of fields, from weather forecasting to predicting financial trends. A computer model first predicted the eastern coast's now famous "Perfect Storm" of 1991. The field of modern fisheries management is highly dependent upon complex computer models of fish population dynamics. For herring and other species, fisheries managers use the models to calculate maximum sustainable yield (MSY) from which a total allowable catch (TAC) is set.

When developing strategies for managing a fish stock, computer modelers incorporate massive amounts of data about the fish species, including biological and environmental parameters and the catch data provided by fishermen. Scientists build mathematical algorithms that organize and analyze the data with the help of powerful computers. The end result is a model of the population dynamics of a particular stock of fish that policy makers and fishery biologists can use to manage fish stocks, hopefully with sustainable results.

That is the goal. With an offshore migratory fish like Atlantic herring, that goal is often elusive. Herring populations in particular are prone to significant fluctuations and mass mortalities. The multitudes of factors that affect the dynamics of a herring's world are extremely complex and variable. Improved models and more accurate data on the herring fishery in the Gulf of Maine may continue to improve the accuracy of these models. In any science, however, the most challenging application of real data is to accurately predict complex future trends.

Overview | Stocks | Stock Differentiation | Management and Computer Models | References

 

References:

[1] Fisheries and Oceans Canada, Northwest Atlantic Fisheries Centre, Atlantic Herring Website. http://www.nwafc.nf.ca/sealane/References/Species/atlantic_herring.htm.

[2] Anthony, V.C. (1981) The use of meristic counts in indicating herring stocks in the Gulf of Maine and adjacent waters. NAFO SCR Doc. 81/IX/127 ser. No. N433: 37.

[3] Oskarsson, G. (7/2002) Personal Communication. Dalhousie University.

[4] McGladdery, S.E., M.D.B. Burt. (1985) Potential of parasites for use as biological indicators of migration, feeding and spawning behavior of northwestern Atlantic herring (Clupea harengus). Can. J. Fish. Aquat. Sci. 42(12): 1957-1968.

[5] Colette, B.B. and G. Klein-MacPhee, eds. (2002) Bigelow and Shroeder's Fishes of the Gulf of Maine, 3rd ed. Smithsonian Institution Press, Washington D.C. 748 pp.

[6] Stephenson, R.L. and I. Kornfield. (1990) Reappearance of Spawning Atlantic herring (Clupea harengus harengus) on Georges Bank: population resurgence not recolonization. Can. J. Fish. Aquat. Sci. 47: 1060-1064.

[7] McPherson, A., C.T. Taggart, and D. Cook. (2000) Microsatellite-Based Analyses of population structure in Atlantic herring. Herring 2000: An international Symposium on Expectations for a New Millennium. Abstracts. University of Alaska Sea Grant College Program, Fairbanks, Alaska.

[8] Phillipi, A. (7/2002) Personal Communication. University of Maine.

 

General References:

New England Fishery Management Council (NEFMC) Herring Stock Assessment and Fishery Evaluation (SAFE) report, 2002 (PDF format) - Current status of herring stocks.

Ransom Myer's Stock Recruitment Database: Herring Stock Recruitment Page.

Marine Research Institute of Iceland, Fish Tagging and Identification

 
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