Visiting Scientists
Dr. Michael G. Fox
Environmental & Resource Studies Program and Department of Biology
Trent University
Peterborough, Ontario, Canada

Karen Hunter
M.Sc. candidate
Watershed Ecosystem Graduate Program
Trent University
Peterborough, Ontario, Canada

Habitat influences on growth and life history traits of the mummichog, Fundulus heteroclitus

Description:

Life history traits often vary markedly among fish populations inhabiting different lakes and streams (see for example, Trippel and Harvey 1987, Reznick and Endler 1982, Hutchings 1993, Fox 1994). In some freshwater and anadromous species, subpopulations inhabiting littoral and limnetic environments within the same lake will form distinct morphotypes that exhibit differences in age and/or size at maturity (Jonsson et al. 1988). While within-population variation has been reasonably well studied in freshwater fishes, salt marsh species have rarely been studied for within-population variation, particularly with respect to life history traits.

The mummichog (Fundulus heteroclitus) is an abundant, highly adaptable fish that inhabits freshwater streams, open estuarine waters and salt marshes along the east coast of the U.S. and Canada (Figure 1). Within salt marshes, mummichogs utilize intertidal and subtidal creeks, as well as pools on the marsh surface. These pools are shallow, highly variable environments, and they act as closed systems except when they are periodically inundated during high tides and storm events. Marsh pools can be extreme environments for fish because of their shallow depth and limited water exchange capacity. While mummichogs are capable of living in these pools, and are often concentrated in them during the winter (Smith and Able 1994), we know very little about how occupation of these pools affects their life histories.

In the spring and summer of 2001 and 2002, we began studying life history variation in mummichogs inhabiting marsh pools and subtidal creeks in the part of the Jacques Cousteau National Estuarine Research Reserve (JCNERR) adjacent to the Rutgers University Marine Field Station. Our objectives were as follows:

(1) To compare the temporal pattern of reproductive allocation in mummichogs inhabiting marsh pools and subtidal creeks

(2) To estimate the annual reproductive effort of individuals inhabiting these respective habitats

(3) To determine whether temporal divergence of reproductive allocation patterns (if any) in pool and creek habitats is associated with temporal changes in temperature regime, salinity, dissolved oxygen concentration and/or somatic growth rates of mummichog

For this study, mummichogs in four pools and four subtidal creeks in the JCNERR (Figure 2) were collected during peak reproductive periods (spring and neap tides) from May to early August. Scales from males and females were taken to age the fish. The scale length increment from the most recent annulus deposited was subtracted from this increment in the previous assessment period to estimate the change in body length between measurement periods. Females were assessed for maturity status, and ovaries were weighed to determine gonadosomatic index (GSI). GSI means for each study site and habitat type (pool or creek) were compared over the spawning period to test for habitat differences in reproductive allocation patterns. Mean GSI during two low points in the reproductive cycle was subtracted from the mean GSI estimate for each peak reproductive period, and the results were summed to develop a conservative estimate of annual reproductive allocation for each study site.

Results from the 2001 field season indicate that mummichogs inhabiting creeks initially allocated more energy to reproduction than those inhabiting pools, and that the reproductive allocation of pool fish declined more drastically over the spawning season and they ceased spawning one reproductive period sooner than the creek fish (Figure 3). Among-pool variability in reproductive allocation during high tide periods was greater in the pools than in the creeks (Figure 4). Estimated annual reproductive allocation was higher in the creek fish as well (Figure 5). Data on seasonal growth patterns indicate that mummichogs inhabiting pools virtually stopped growing in early July; the same time that their reproduction ceased (Figure 6). Creek fish did not show this drastic reduction in growth rate, but nevertheless ceased reproduction in mid-July. While these data suggest that mummichogs inhabiting pools reduce their late season allocation to reproduction because of energy limitation, spot temperature measurements taken in the study sites also show that late season temperatures were higher in pools than creeks; thus higher metabolic requirements associated with warmer water temperatures may play a role in the pool-creek differences in reproductive allocation.

In the 2002 field season, growth and reproduction data were again collected on the eight study sites. In addition, we continuously monitored water temperature in the study sites, and took early-morning dissolved oxygen measurements weekly during the spawning season. To get a clearer picture of population dynamics, mortality rates and movement of mummichogs in and out of pools, we also did an intensive mark-recapture study in six additional pools. Two of these pools were fenced with net enclosures to prevent fish movement, and are being used to estimate natural mortality rates

Further information on the life history research of Michael Fox and his students can befound at the following web site: http://www.trentu.ca/ers/Fox.shtml.

Literature Cited

Fox, M.G. 1994. Growth, density, and interspecific influences on pumpkinseed sunfish life histories. Ecology 75: 1157-1171

Hutchings, J.A. 1993. Adaptive life histories effected by age-specific survival and growth rate. Ecology 74: 673-684.

Jonsson, B., S. Skulason,, SS. Snorrason, O.T. Sandlund, H.J. Malmquist, P.M. Jonasson, R. Gydemo and T. Lindem. 1988. Life history variation of polymorphic Arctic charr (Salvelinus alpinus) in Thingvallavatn, Iceland. Canadian Journal of Fisheries and Aquatic Sciences 45: 1537-1547.

Reznick, D.A. and J.A. Endler. 1982. The impact of predation on life history evolution in Trinidadian guppies (Poecilia reticulata). Evolution 36: 160-177.

Smith, K.J. and K.W. Able. 1994. Salt-marsh tide pools as winter refuges for the mummichog, Fundulus heteroclitus, in New Jersey. Estuaries 17:226-234.

Trippel, E.A. and H.H. Harvey. 1987. Reproductive responses of five white sucker (Catostomus commersoni) populations in relation to lake acidity. Canadian Journal of Fisheries and Aquatic Sciences 44: 1018-1023.