Life History Research Program
Current Research and Future Directions
The purpose of the life history program at the Perry R. Bass Marine Fisheries Research Station is to investigate sport fish population and life history parameters that cannot be adequately addressed using routine monitoring data. Studies may be coast-wide, or may be restricted to a single bay system. For the past ten years much of the work has concentrated on age, growth, and the development of age-length keys which are used in population assessments. Other areas receiving considerable attention involve reproductive biology, stocking program evaluation, special problems, and a cooperative fish ageing program with the Gulf States Marine Fisheries Commission (GSMFC).
Age and Growth:
Historically, fish age has been determined using a variety of techniques including length frequency histograms, scales, and mark-recapture studies (Jearld 1983). More recently, fish otoliths have been recognized as the most accurate method of determining fish age (Beamish and McFarlane 1987). Accuracy in ageing is critical to developing management strategies for sustainable fisheries; underestimating age results in overestimating production and can lead to management that appears to be appropriate but is actually allowing overfishing (Beamish and McFarlane 1987). Analysis of otoliths removed from fish collected during routine monitoring is the primary method used for ageing fish at the PRBMFRS life history laboratory. Otoliths are embedded in resin and thin sectioned using a high-speed diamond bladed saw. Sections are mounted on microscope slides and distances from the center of the otolith to each age ring, or annulus, and to the otolith margin are recorded using an optical imaging system. Typically, plots of the marginal increment (the distance from the last annulus to the edge of the otolith section) are used to confirm that age rings are produced once each year (validation) for each species aged. A number of ageing projects have been conducted at PRBMFRS; ranging from studies on red drum, black drum, spotted seatrout, red snapper, and southern flounder to the examination of otoliths from archaeological digs in coastal Native American middens. Summaries of the completed and ongoing projects are as follows:
Coast-wide studies of red drum age and growth were conducted from 1993 through 1995 (Colura and Buckley 1996). Age-length keys were constructed quarterly and semi-annually and differences between them evaluated. At least semi-annual revision was found to be necessary if age-length keys are to be used for age-structured population assessments. Von Bertalanffy growth models fitted for each bay differed among bays, with only two exceptions. Within bay growth of Texas red drum is slower than growth rates reported for Florida and North Carolina red drum. However, the models predicted similar lengths at age and a single coast-wide von Bertalanffy growth model which includes mature fish from offshore was recommended for use in management of Texas red drum.
Spotted seatrout investigations revealed that growth varies among bays and years (Colura et al. 1994) suggesting frequent monitoring of growth is necessary. These findings precipitated current studies to determine the frequency at which age-length keys must be revised for use in age structured stock assessments.
Von Bertalanffy growth models revealed that growth differs among bays. Mean measured size at age for upper Laguna Madre fish was significantly smaller than size of fish from other bays, with a maximum differential of 109 mm at 4.3 years of age.
Growth models underestimated length of younger fish and overestimated lengths of older fish probably because few older fish were in the sample.
Differences in growth between male and female flounder were found, males are smaller than females at the same age. Additionally, the ratio of males to females coast-wide was 1:6.
Female Atlantic croaker grew faster than males, and had a larger estimated maximum length. Only 4% of the fish collected were more than 2 years old. Growth was highly variable.
Remains of fishes, including otoliths, frequently constitute a major component of coastal archaeological sites. Examination of otoliths recovered from archaeological sites provide insights into the biology of prehistoric populations of fishes. For instance, examination of otoliths of Atlantic croaker (Micropogonias undulatus) and sea catfishes (Arius felis and Bagre marinus) recovered from archaeological sites in the southeastern United States demonstrated changes in biology (age and growth) of these fishes over relatively short periods of time (~500 years) (Hales and Reitz 1993). Atlantic croaker as old as age 15 were found in association with approximately 500 year old Spanish archaeological sites in Florida (Hales and Reitz 1992). Presently, Atlantic croaker older then age 2 are seldom found (White and Chittenden 1977). Changes in age structure between ancient and contemporary populations are believed to be the result of fishing (Hales and Reitz 1993). Examination of otoliths from archaeologicalsites has also suggested changes in growth rates have occurred. Noguera (1994) examined a small number of red drum (Sciaenops ocellatus) (n=83) and black drum (Pogonias cromis) (n=100) from a Texas archaeological site (~1000 years old) and found growth differed from modern populations and hypothesized the difference was due to overfishing.
Total mortality is the sum of natural and fishing mortality (Ricker 1975). Total mortality of fishes, in general, is estimated by measuring the differences in frequency of occurrence in a sample of younger and older age groups (age groups are assigned by examining otoliths, or scales, etc). Numerous methods have been developed to estimate natural mortality. The most widely accepted methods use von Bertalanffy growth parameters to make the estimate (Hoenig 1983, Pauly 1980). However; if, due to fishing, older age groups have been removed from the population, use of the von Bertalanffy growth parameters will result in estimates of natural mortality which are greater than the true rate of natural mortality. As such, estimates of fishing mortality (fishing mortality=total mortality-natural mortality) are underestimated and can result in adoption of regulations which cause overfishing. Over estimation of natural mortality because older age groups have been removed from the population has created problems with management of marine fishes such as red snapper (Gregory et al. 1995). Total mortality of unfished or lightly fished populations approximates natural mortality (Hoenig 1983).
Fishery managers can use estimates of total mortality of lightly or unexploited populations to determine if estimates of natural mortality for heavily fished populations of the same species are reasonable. Unfortunately, few, if any, unfished or lightly exploited contemporary populations of sportfishes exist. Otoliths recovered from archeological sites, however, are from fish populations that are presumed to have been lightly exploited and age data obtained through examination of the otoliths may be used to estimate natural mortality. During recent archaeological investigations on the central Texas coast (Rickless 1993) more than 2000 otoliths of red drum, spotted seatrout, black drum and Atlantic croaker were recovered from strata dating from 5000-800 years BP (before present). The otoliths were made available to the Texas Parks and Wildlife, Coastal Fisheries Branch. Estimates of total length, age distribution and mortality in ancient Texas bays have been completed and are as follows:
Age at maturity, spawning frequency, fecundity, and spawning season are important in determining the reproductive characteristics of a species. The gonadosomatic index (GSI), which is the percentage of the body weight represented by the gonad, is used as an indicator of maturity and to determine the spawning season. The spawning potential ratio (SPR) of a species is a measurement of the average number of eggs each recruit would produce in an exploited condition versus an unexploited situation, and is used as a guideline to determine the effect of fishing on reproduction in a fish population. All of the above factors must be considered in determining the amount of fishing pressure a species can withstand without declining. The reproductive biology of several species has been examined by PRBMFRS staff, including black drum, spotted seatrout, red snapper, and flounder. Some completed and ongoing reproductive biology studies are summarized below.
Black drum life history has been investigated by the life history group because of disparate reports on the age at maturity of black drum. Reports from Texas (Pearson 1929, Simmons and Breuer 1956) indicated black drum began spawning at two years in the upper Laguna Madre, while studies from the eastern Gulf of Mexico (Murphy and Taylor 1989, Nieland and Wilson 1993) reported spawning did not begin until fish were 5 years old. We found that a portion of the black drum in the upper Laguna Madre begin spawning at two years of age, while fish from other bay systems did not mature until they were 5 years old. We also found different growth rates for black drum in the upper Laguna Madre and the rest of Texas bays, probably due to the difference in age at maturity.
The reproductive biology of spotted seatrout has been the subject of several projects at PRBMFRS, initial projects dealt with induced spawning of spotted seatrout (Colura 1974, Colura et al. 1988, Colura et al. 1990). Physiological studies on the effect of temperature and salinity on growth of juveniles and hatching success of eggs were also completed (Bumguardner and Maciorowski 1989, Gray et al. 1991). Additional studies have investigated the portion of the age 1 year class that are mature and their reproductive capacity, and the reproductive response of spotted seatrout to mass mortality events. The life history group at PRBMFRS collected spotted seatrout less than 380 mm total length from the upper middle and lower Texas coast in early and late summer to evaluate the reproductive potential of age 1 spotted seatrout. Otoliths were collected to confirm age of all fish, and gonad weights and samples were taken to determine the reproductive stage of each individual collected. Information on the spawning capability of age 1 spotted seatrout is important in determining the spawning potential ratio (SPR) for spotted seatrout. The PRB life history group also compared the reproductive characteristics of spotted seatrout in the lower Laguna Madre, which underwent massive cold mortality in January, 1997 to spotted seatrout in Matagorda Bay, which were unaffected by the cold weather. Current studies on spotted seatrout involve the use of hydrophones to locate spawning aggregations and characterize spawning habitat.
Reproductive biology studies of red snapper revealed that about 37-48% of 2 and 3 year old red snapper spawn but for a much shorter period than fish 4 years of age and older. All fish 4 years of age and older were mature (Bumguardner et al. 1996).
Most Southern flounder were mature at two years and 350 mm total length (females), and all southern flounder were mature at 3 years. Sex ratio of male to female fish was found to be 1:6, coastwide. GSI values indicate gonadal development begins in September, and gonads were well developed by January (Stuntz et al. 1996).
We are currently collecting fish from all bay systems and offshore areas and will determine spawning season and age at maturity for Atlantic croaker.
Stocking Program evaluation:
The red drum stocking program operated by TPWD releases 20-30 million red drum fingerlings annually (McEachron et al. 1998). The life history staff has conducted two programs designed to evaluate the effectiveness of red drum stocking. The methods used are comparison of lengths of red drum caught in gill nets in stocked and unstocked bays, and chemical marking of fish released in selected bays.Length Comparison A four year evaluation of the red drum stocking program was begun in 1996. Samples of red drum collected from Cedar Lakes, an unstocked area located near the mouth of the San Bernard River, are being compared to those collected from stocked bays. Results suggest some bays stocked in the spring are being impacted by the stocking program (McEachron et al. 1998). Chemical Marking Chemical marking of red drum has been conducted by the life history group at PRBMFRS as a method of identifying hatchery reared red drum released in Texas bays. Red drum juveniles (~25 mm total length) were immersed in an oxytetracycline solution. The oxytetracycline (an antibiotic known by the trade name terramyacin@) binds with calcium in the bloodstream and is incorporated into bone with the calcium. When viewed under ultraviolet light the calcium-oxytetracycline complex produces a bright yellow-green fluorescence. Red drum marked with OTC have been stocked in the upper Laguna Madre and in West Galveston Bay. Red drum caught in TPW gillnets and by sport anglers in these two areas are monitored for chemical marks.
The life history laboratory at PRBMFRS also deals with special studies that are outside the normal duties of the TPW field station personnel, or that require specialized capabilities. Examples of studies that fall into this category are questions on: the effects of salt-boxes on bycatch survival, the diameter escape rings in crab pots should be to allow undersize crabs to escape but still retain legal crabs, and the distribution and movement of larval, juvenile, sub-adult, and adult tarpon in Texas waters.Salt-Box Use Use of salt-boxes by the Texas bay shrimping industry to separate bycatch from shrimp was studied in 1995. Salt-boxes were found to be used most often in Matagorda and San Antonio Bays where 90% and 100% of the fishermen interviewed in each bay, respectively, reported using saltboxes. Zero to 58% of shrimp fishermen interviewed in other Texas bays reported using saltboxes. Most salt-boxes were made of wood and with dimensions ranging from 0.6 m x 0.6 m x 0.9 m to 1.2m x 2.4m x 0.9m. Bioassays conducted on 5 economically important species (spotted seatrout (Cynoscion nebulosus), red drum (Sciaenops ocellatus), Atlantic croaker (Micropogonias undulatus), southern flounder (Paralichthyes lethostigma), and blue crab (Callinectes sapius)) frequently or occasionally exposed to hypersaline conditions in saltboxes found red drum were most affected by hypersalinity. Red drum required 17 minutes exposure to a 70 0/00 salt water solution to kill 50% of the test specimens within 48 hours. Analyses of bycatch survival data, both from samples collected from commercial boats and Texas Parks and Wildlife (TPW) trawl samples, found survival is statistically similar for bycatch removed using a saltbox and bycatch removed without the aid of a saltbox. However, bycatch mortality is high regardless of the method used to separate bycatch. At the conclusion of catch separation, mean plus/minus SD survival of bycatch separated using saltboxes was 76 plus/minus 22% for samples collected from commercial vessels and 48 plus/minus 40% for TPW trawl samples. Mean plus/minus SD survival at the conclusion of catch separation without a saltbox were 56 plus/minus 35% for samples collected from commercial vessels and 43 plus/minus 39% in TPW trawl samples. Results of exploratory analysis using stepwise multiple regression suggests survival of bycatch is most affected by the time required to separate the catch (total time on deck) and length of time the trawl was fished. Tarpon distribution Cast net samples were been collected in all bay systems for three years, icthyoplankton samples have been collected at the Port Aransas jetties, volunteer anglers have been recruited to tag adult tarpon and GIS maps have been constructed of all known larval juvenile and adult tarpon collection sites. Eight juvenile tarpon were collected in 3,445 cast net samples at 713 sites from eight bay systems. Published reports of juvenile tarpon cold tolerance in fresh water (Howells 1985) of 9.5 to 18.2oC suggest Texas waters may be marginal, at best, for sustained tarpon recruitment.
GSMFC Cooperative Program:
A fish ageing program dealing with collecting, processing and ageing otoliths from commercial and recreational sources for selected marine species has been administered by TPWD staff at PRBMFRS and the Rockport regional office since 2002. Species currently collected and aged are red snapper, southern flounder, king mackerel and greater amberjack. Approximately 3,600 otoliths are collected and processed annually by GSMFC contract employees.
Future areas of research involving the life history staff at the PRBMFRS include continuation of ongoing research on spotted seatrout reproduction and age-length keys, red drum age length keys, evaluation of stocking programs, and tarpon distribution and tagging. Results of these studies will be used to evaluate and refine current management strategies. Potential new projects could involve studies on life history of species such as sheepshead, tripletail, snook and grey snapper to provide information needed to formulate management strategies for these and other species. Additionally, further evaluation of Atlantic croaker and southern flounder growth curves and age and length distribution may be useful for evaluating the impact of management regulations requiring use of by-catch reduction devices in bay and gulf shrimp trawls.
Perry R. Bass Marine Fisheries Research Station