NEFSC Fishery Biology Program

Fish Reproduction

I. History of fish reproduction research in Woods Hole

Fisheries are renewable resources and reproduction completes this circle of life. Some of the earliest research associated with the U.S. government's Woods Hole Laboratory focused on fish reproduction (Bumpus, 1898) and fish culture (Hobart, 1996). Early research in Woods Hole investigated the eggs and larvae of regional fishes (Fish, 1925, 1928) and invertebrates (Galtsoff, 1930). As a specific example, Henry Van Wilson described fertilization of the black sea bass egg and its development from hatching to free-swimming larva (Wilson, 1891).

We continue this legacy of applied and basic research of marine fish reproduction at the NEFSC. As part of our mission, the Population Biology Branch develops products to estimate how fish maturity is a function of fish size and age. We also investigate how individual, population, and ecosystem processes determine fish fecundity, that is the number of eggs produced. Read on to learn more.

Citations used in section I.

  1. Bumpus, H. C. 1898. The breeding of animals at Woods Hole during the month of May, 1898. Science N.S. 8(185): 58-61.

  2. Fish, C. J. 1925. Seasonal distribution of the plankton of the Woods Hole region. Bulletin of the United States Bureau of Fisheries XLI: 91-179.

  3. Fish, C. J. 1928. Production and distribution of cod eggs in Massachusetts Bay in 1924 and 1925. Bulletin of the United States Bureau of Fisheries 43(2): 253-296.

  4. Galtsoff, P. S. 1930. The fecundity of the oyster. Science 72(1856): 97-98.

  5. Hobart, W. L. 1996. Baird's Legacy: The history and accomplishments of NOAA's National Marine Fisheries Service, 1871-1996. NOAA Technical Memorandum NMFS-F/SPO-18. (HTML version here.)

  6. Wilson, H. V. 1891. The embryology of the sea-bass (Serranus atrarius). Bulletin U.S. Fish Commission (for 1889) 9: 209-277.

II. How do fish reproduce?

No simple generalizations can describe how all marine fish reproduce. Most marine fish produce thousands of small eggs per spawning event; however some species produce far fewer but much larger eggs per spawning event. Most marine fish broadcast these eggs into the water to be fertilized externally; however many species lay eggs in nests on the benthic substrate, and some of these will even guard the eggs or the newly hatched young. Other fishes fertilize their eggs internally, provide nourishment to the developing embryos, and release live young after a period of gestation. Although many species have separate sexes, individuals of some species change sex or may have the capacity to produce spermatozoa and eggs at the same time. So, although females of most marine species produce lots of small eggs for external fertilization, this is not a universal strategy (f. ex., Murua and Saborido-Rey, 2003).

The simplest but most uncommon reproductive life cycle is to spawn once and die. Such fish, called semelparous, typically experience an exhausting spawning migration from which they cannot recover (Figure 1). Semelparous species adapt to this apparent disadvantage by producing more eggs per individual in a single spawning event than closely-related species that spawn more than once in a lifetime.

The term iteroparous describes fish that spawn over multiple annual seasons during their life. Females of some iteroparous fishes release their eggs in a single wave of spawning (total spawners), just like semelparous species, but they have the capability to spawn in future years as well (Figure 2).

More commonly, iteroparous fish spawn multiple times during the spawning season (Figure 3). The annual fecundity of these batch spawners may be very high since the potential maximum number of eggs per year is not limited by the number that can be carried in the female's body cavity at one time.

Reproduction is a very energy-intensive event, and once a fish matures, it does not necessarily spawn in every year (Figure 4). Many fishes are known to use skip spawning in a given year when energy is insufficient to complete production of gametes (eggs, sperm). This may occur most commonly in newly mature fish or in a year when ecosystem productivity is lower than average. When it occurs, the number of mature fish is not the same as the number of spawning fish.

The reproductive cycle of some fishes may be complicated by sex change occurring among hermaphrodites. Sex change can occur in many species, although it is often clustered in specific families of fishes. Many members of the sea bass or grouper family (Serranidae) begin their reproductive life as females and change sex to males later in life (Figure 5). This occurs when a single, large male can monopolize all spawning activities in a local area, often accomplished by establishing and defending a haremic mating system.

Citations used in section II.

  1. Burnett, J., L. O'Brien, R. K. Mayo, J. A. Darde and M. Bohan. 1989. Finfish maturity sampling and classification schemes used during Northeast Fisheries Center bottom trawl surveys, 1963-89. NOAA Technical Memorandum NMFS-F/NEC-76: 14 pp.

  2. Drohan, A. F., J. P. Manderson and D. B. Packer. 2007. Essential Fish Habitat Source Document: Black sea bass, Centropristis striata, life history and habitat characteristics (2nd Edition). NOAA Technical Memorandum NMFS-NE-200: 68 pp.

  3. Fahay, M. P. 1978. Biological and Fisheries Data on American eel, Anguilla rostrata (LeSueur). Sandy Hook Laboratory, Northeast Fisheries Center, National Marine Fisheries Service, National Oceanic and Atmospheric Administration, U. S. Department of Commerce, Highlands, N. J. Technical Series Report No. 17: 96 pp.

  4. Lough, R. G. 2004. Essential Fish Habitat Source Document: Atlantic cod, Gadus morhua, life history and habitat characteristics (2nd Edition). NOAA Technical Memorandum NMFS-NE-190: 94 pp.

  5. Murua, H. and F. Saborido-Rey. 2003. Female reproductive strategies of marine fish species of the North Atlantic. Journal of Northwest Atlantic Fishery Science 33: 23-31.

  6. Pereira, J. J., R. Goldberg, J. J. Ziskowski, P. L. Berrien, W. W. Morse and D. L. Johnson. 1999. Essential Fish Habitat Source Document: Winter flounder, Pseudopleuronectes americanus, life history and habitat characteristics. NOAA Technical Memorandum NMFS-NE-138: 39 pp.

  7. Stevenson, D. K. and M. L. Scott. 2005. Essential Fish Habitat Source Document: Atlantic herring, Clupea harengus, life history and habitat characteristics (2nd edition). NOAA Technical Memorandum NMFS-NE-192: 84 pp.

american eel
Figure 1. Semelparity.
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atlantic herring
Figure 2. Total Spawner.
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atlantic cod
Figure 3. Batch Spawner.
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winter flounder
Figure 4. Skip Spawner.
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black sea bass
Figure 5. Hermaphrodite.
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III. When do fish begin to reproduce?

Some fish species mature as early their first year, whereas others delay maturity for over a decade. The time to maturity says a great deal about the productivity of a population and its ability to rebound from environmental disturbance or high fishing rates. Because fishing rates are typically set to retain a certain percentage of the spawning females in a population, monitoring size and age at maturity is common and these are important parameters used in stock assessment (O'Brien et al. 1993). Here, the general appearance of the female gonad, the ovary, and six major time steps of maturity are illustrated for winter flounder to describe how this monitoring typically occurs (see maturity cycle, right).

For cost reasons, maturity is typically classified by a macroscopic method, examining characters such as gonad size, color, texture, or shape using fresh, dissected fish at sea. The flounder ovary consists of two lobes, like most fish, but these lobes are separated by the lower (hemal) spines so that only the right ovary lobe is exposed in these images. As the ovary matures, it grows in length from the anterior end of the body cavity towards the tail. With growth, the gonad broadens (dorso-ventral) at the anterior end and tapers to a point at the posterior end so that each lobe becomes an elongated triangle.

All females begin immature. Key characteristics that indentify immature winter flounder gonads are: 1) small size (gonad length is less than 30% of the fish's total length); 2) no or few blood vessels, and if present these are very thin; 3) a thin gonad outer membrane (< 0.1 mm thick) which tears easily; 4) gonad cavity itself is compact and has not yet extended far beyond the ovary (once the fish matures the cavity will extend to the tail, creating an empty space in a resting fish). Color is a helpful character with one qualification. Initially the immature gonad is clear, without color, so the individual is easy to identify as immature. Later, when the gonad begins to mature, but months before it spawns for the first time, the immature gonad becomes orange but remains translucent (semi-transparent). Thus, the simple presence of color does not automatically mean the fish is mature.

Once a fish matures, it remains mature. Winter flounder are iteroparous, which means once they mature they are capable of spawning in more than one year. Therefore, a mature fish cycles between a spawning period and a non-spawning period. Prior to spawning, 'developing' females provision their developing eggs with yolk, which creates a uniform yellow color to the gonad. During the spawning period, females are referred to as 'ripe' when they have nearly-mature eggs scattered throughout their gonad and 'running ripe' once these mature eggs have ovulated and are ready for release. Immediately following spawning, females are considered 'spent', later on 'resting', and still later they become 'developing' again.

There is also evidence that winter flounder can mature but then not spawn in every subsequent year. 'Skipped' spawners will look like 'Resting' fish using the scheme outlined above. Skipped spawning is important because when it happens, such mature fish are not part of the spawning component of the population. Research at the NEFSC has shown that skipped spawning occurs in < 5% of the mature females in U.S. populations. Skipped spawning is much higher in Canadian stocks of winter flounder, as much as 20%, where the growing season is much shorter so there is less energy available for reproduction (Rideout et al. 2011).

How do we know these simple visual descriptions are correct? Scientists in the Population Biology Branch have processed gonad tissue using histology to validate our at-sea classifications of maturity. Histology is a laboratory technique that fixes tissue with chemicals so that it can be sliced very thin and stained for viewing under the microscope. By doing so, we can compare of the initial, macroscopic appearance of gonads to the microscopic, detailed images of cellular structure revealed by gonad histology.

The photographic plate (Fig. 6) depicts paired macroscopic and microscopic images of three female winter flounder to illustrate the transitions from:
1) immature and not ready to spawn for more than a year, to
2) immature but developing to spawn the following spring, and
3) mature but resting during the non-spawning season.

All three fish had only primary (unyolked) oocytes, but the mature female had a thick gonad wall (arrow), an indication of past spawning. Comparisons of macroscopic and microscopic classification is presented in McBride et al. (2012), and details of winter flounder oogenesis were presented by Press (Rowinski) et al. (2010) at the 12th Flatfish Biology Conference.

Citations used in section III.
  1. O'Brien, L., J. Burnett and R. K. Mayo. 1993. Maturation of nineteen species of finfish off the northeast coast of the United States, 1985-1990. NOAA Technical Report, National Marine Fisheries Service, Special Scientific Report - Fisheries Series 113: 1-66.

  2. McBride, R. S., M. J. Wuenschel, P. Nitschke, G. Thornton, and J. R. King. 2013. Latitudinal and stock-specific variation in size- and age-at-maturity of female winter flounder, Pseudopleuronectes americanus, as determined with gonad histology. Journal of Sea Research (Special Issue: Proceedings of the 8th International Symposium on Flatfish Ecology, Part I) 75: 41-51.

  3. Rideout, R. M. and J. Tomkiewicz. 2011. Skipped spawning in fishes: more common than you might think. Marine and Coastal Fisheries: Dynamics, Management, and Ecosystem Science 3: 176-189.

  4. Press (Rowinski), Y., R. McBride, M. Wuenschel and D. McElroy. 2010. A histological atlas of female winter flounder (Pseudopleuronectes americanus) oogenesis. Twelfth Flatfish Biology Conference 2010 Program and Abstracts December 1-2, 2010, Water's Edge Resort and Spa, Westbrook CT. Northeast Fisheries Science Center Reference Document 10-21: 77pp.

maturity cycle
Maturity Cycle
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running ripe
Running Ripe
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microscopic images
Figure 6. Comparison
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IV. How many eggs do fish produce?

Once a female matures, she is capable of producing eggs. In many population models, it is assumed that the number of eggs is proportional to fish size, particularly fish weight. Although this is a reasonable first approximation of how the rate of egg production changes over the lifespan of a fish, the rate of egg production can vary by many other factors, such as fish age or habitat quality. This is also true for the quality of the eggs, such that fertilization rates or hatching success may vary due to 'maternal' effects.

The Population Biology Branch is investigating factors like maturity that may affect fish fecundity. To do this, we need to count fish eggs, which can be a very labor-intensive process. In the 1960s, the Woods Hole Laboratory used a photoelectric egg counter (right top) to speed up this process.

Today, image analysis software is used to automate the counting of eggs from samples of gonad tissue (Witthames et al. 2009). First, oocyte sizes are measured using ImageJ software (right middle) and are calibrated to number of eggs per gram of ovary tissue (right bottom). Later, only oocyte size is measured and oocyte densities are expanded by the gonad size to estimate the potential number of eggs produced by each female. The NEFSC is applying this to a number of groundfish species, such as winter flounder, to demonstrate that reproductive rates vary by region and between years (McElroy et al. 2012).

Citations used in section IV.

  1. Graham, H. W. 1962. Annual Report of the Bureau of Commercial Fisheries Biological Laboratory, Woods Hole, Massachusetts. Circular 137.

  2. McElroy, W. David, Mark J. Wuenschel, Yvonna K. Press, Emilee K. Towle, and Richard S. McBride. 2013. Differences in female individual reproductive potential among three stocks of winter flounder, Pseudopleuronectes americanus. Journal of Sea Research (Special Issue: Proceedings of the 8th International Symposium on Flatfish Ecology, Part I) 75: 52-61.

  3. Witthames, P. R., L. N. Greenwood, A. Thorsen, R. Dominguez, H. Murua, M. Korta, F. Saborido-Rey and O. S. Kjesbu. 2009. Advances in methods for determining fecundity: application of the new methods to some marine fishes. Fishery Bulletin, U. S. 107: 148-164.

Photoelectric Counter
Photoelectric Counter
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New Egg Counter
Using ImageJ Software
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Density Vs Diameter
Eggs per Gram
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V. Recent products by the NEFSC about fish reproduction

Recent Publications
  1. McBride, Richard S., Stylianos Somarakis, Gary R. Fitzhugh, Anu Albert, Nathalia A. Yaragina, Mark J. Wuenschel, Alexandre Alonso-Fernández, and Gualtiero Basilone. 2015. Energy acquisition and allocation to egg production in relation to fish reproductive strategies. Fish and Fisheries 16(1): 23-57.

  2. Collins, Angela B., and Richard S. McBride. 2015. Variations in reproductive potential between nearshore and offshore spawning contingents of hogfish in the eastern Gulf of Mexico. Fisheries Management and Ecology 22(2): 113-124.

  3. Press, Yvonna K., Richard S. McBride, and Mark J. Wuenschel. 2014. Time course of oocyte development in winter flounder (Pseudopleuronectes americanus) and spawning seasonality for the Gulf of Maine, Georges Bank, and Southern New England stocks. Journal of Fish Biology 85(2): 421-445.

  4. Winton, Megan V., Mark J. Wuenschel, and Richard S. McBride. 2014. Investigating spatial variation and temperature effects on maturity of female winter flounder (Pseudopleuronectes americanus) using generalized additive models. Canadian Journal of Fisheries and Aquatic Sciences 71: 1279-1290.

  5. Hyle, R., R. McBride, and J. Olney. 2014. Determinate versus indeterminate fecundity of American shad, an anadromous clupeid. Transactions of the American Fisheries Society 143(2): 618-633.

  6. McBride, Richard S., Vidal, Tiffany E., and Steven X. Cadrin. 2013. Changes in size and age at maturity by the northern stock of tilefish (Lopholatilus chamaeleonticeps) following a period of overfishing. Fishery Bulletin 111: 161-174.

  7. McBride, Richard S., Mark J. Wuenschel, Paul Nitschke, Grace Thornton, and Jeremy R. King. 2013. Latitudinal and stock-specific variation in size- and age-at-maturity of female winter flounder, Pseudopleuronectes americanus, as determined with gonad histology. Journal of Sea Research (Special Issue: Proceedings of the 8th International Symposium on Flatfish Ecology, Part I) 75: 41-51.

  8. McElroy, W. David, Mark J. Wuenschel, Yvonna K. Press, Emilee K. Towle, and Richard S. McBride. 2013. Differences in female individual reproductive potential among three stocks of winter flounder, Pseudopleuronectes americanus. Journal of Sea Research (Special Issue: Proceedings of the 8th International Symposium on Flatfish Ecology, Part I) 75: 52-61.

  9. Wuenschel, Mark J., Richard S. McBride, and Gary R Fitzhugh. 2013. Relations between total gonad energy and physiological measures of condition in the period leading up to spawning: Results of a laboratory experiment on black sea bass (Centropristis striata). Fisheries Research (Special Issue: Fish Reproduction) 138: 110-119.

  10. McBride, Richard S., Derke J. G. Snodgrass, Douglas H. Adams, Steven J. Rider, and James A. Colvocoresses. 2012. An indeterminate model to estimate egg production of the highly iteroparous and fecund fish, dolphinfish (Coryphaena hippurus). Bulletin of Marine Science 88: 283-303.
Recent Presentations
  1. Ms. Emilee Towle presented a poster entitled 'Seasonal patterns of oogenesis and spawning of female Yellowtail Flounder (Limanda ferruginea) in the Gulf of Maine: defining a period to measure potential annual fecundity' at the 2014 Winter Science Meeting of the Southern New England Chapter, American Fisheries Society, held in Hadley, MA.

  2. Dr. Richard McBride presented a poster entitled 'Beyond ‘Flatland’: using gonad histology to classify female winter flounder reproductive status' at the 2014 Winter Science Meeting of the Southern New England Chapter, American Fisheries Society, held in Hadley, MA.

  3. Dr. Richard McBride presented the keynote address for the symposium, 'Understanding reproductive dynamics of marine fishes to inform fishery management' at the 35th Annual Larval Fish Conference organized by the Early Life History Section of the American Fisheries Society, 22-26 May 2012, held in Wilmington, NC.

  4. Dr. Richard McBride presented 'The biological basis of egg production and implications for stock assessments' at the 2008 National Stock Assessment Workshop held in Townsend, WA, in the theme session 'Incorporating Reproductive Biology in Stock Assessments.'

  5. Ms. Yvonna Press (Rowinski) presented a poster entitled 'A histological atlas of female winter flounder (Pseudopleuronectes americanus) oogenesis' at the 12th Flatfish Biology Conference, 1-2 December 2010, held in Westbrook, CT.

VI. Other links

The following internet links provide useful information about recent or on-going research regarding fish reproductive biology.
  1. Journal of Northwest Atlantic Fishery Science special issue on "Reproductive Potential of Fish Populations of the North Atlantic" (

  2. Journal of Northwest Atlantic Fishery Science special issue on "Reproductive and Recruitment Processes of Exploited Marine Fish Stocks" (

  3. FRESH (Fish Reproduction and Fisheries) network of researchers (

  4. "Maturity and spawning in fish" flyer by the Irish Marine Institute ( and Spawning in Fish.pdf)

  5. Science Daily article about use of herring reproduction in fishery management (

  6. Maturity schema used by other laboratories:

    1. ICES, (International Council for the Exploration of the Sea). 2007. "Report of the workshop on sexual maturity sampling (WKMAT), 15-19 January 2007, Lisbon, Portugal." ICES CM 2007/ACFM:03: 85. (

    2. Brown-Peterson, N. J., D. M. Wyanski, F. Saborido-Rey, B. J. Macewicz and S. K. Lowerre-Barbieri. 2011. "A standardized terminology for describing reproductive development in fishes." Marine and Coastal Fisheries: Dynamics, Management, and Ecosystem Science 3: 52-70. (

    3. Núñez, J., and F. Duponchelle. 2009. Towards a universal scale to assess sexual maturation and related life history traits in oviparous teleost fishes. Fish Physiology and Biochemistry 35(1):167-180. (

  • Quantitative methods for estimating maturity:

  • ICES (International Council for the Exploration of the Sea). 2008. "Report of the workshop on maturity ogive estimation for stock assessment (WKMOG), 3-6 June 2008, Lisbon, Portugal." ICES CM 2008/ACOM:33: 55, + 51 appendix. ( Reports/Expert Group Report/acom/2008/WKMOG/WKMOG08.pdf)
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