Fisheries Biology - Reproduction
Providing valuable information to fish stock assessment models.
The study of reproductive life history provides valuable information to fish stock assessment models including:
- Sex ratio
- Fecundity estimates
- Reproductive health
- Gonadal development
The Panama City Laboratory studies the reproductive biology of teleosts (ray-finned fishes) collected in the Gulf of Mexico with the collaboration of fishery sampling programs across NOAA Fisheries including the Galveston, Mississippi Observer Program, the Shark Bottom Longline Observer Program, the Panama City Laboratory’s Survey of Recreational Landings, the Pascagoula, Mississippi Laboratory, the Panama City Laboratory Reef Fish Survey, as well as other cooperative projects.
The compiled data supports the needs of the Southeast Data Assessment and Review teams. Our lab works with partners to improve the standardization of methods and deliver information needed to effectively manage our fisheries. Hence, processing priorities align with SEDAR schedules and special projects.
Reproductive information, along with age and growth estimates, provide the fundamental basis for assessing the condition and resiliency of a fish stock. Much of our focus is on fishes of high economic value for the Gulf region including snappers, groupers, and migratory species (e.g., mackerels and tuna).
Size-at-maturity, the size or age at which individuals are reproductively active and producing offspring is often used in stock assessments. It is a fundamental principle of fisheries management to allow individuals to grow to maturity before allowing substantial harvest. It is often found that as individuals increase in size and age, they are more likely to be reproductively active, have more viable eggs—thus it is particularly important to measure this relationship. Maturity, together with sex ratio, allows an estimate of the fraction of the stock likely to be producing eggs. Among the species we study, groupers and porgies are interesting and unique from a reproductive aspect because they change sex (hermaphrodites); one pattern by beginning life as females, then switching to males (protogynous hermaphrodites). Thus, for these species it is also important to determine the size and age relationship corresponding to the transition from female to male.
Another important element to define reproductive potential includes fecundity, the numbers of ova that may be spawned in a given season. In subtropical waters such as the Gulf of Mexico, spawning seasons can be very protracted over many months or even the majority of the year and fishes are commonly multiple- or batch spawners. The fecundity pattern would be characterized as indeterminate and the batch size (clutch of yolked ova at a particular sampling time) and spawning frequency, based upon size or age of the fish, must be determined to make annual fecundity estimates. However, some species show traits more common to cold water species such as a brief spawning period with a determinate pattern of fecundity requiring a different methodology. Their fecundity is “determined” before the onset of spawning timed to seasonal changes. As such, there is a need to categorize and verify these patterns before the process of counting ova begins.
Processing gonads using Panama City Lab histological guidelines (from top to bottom):
- Gonads are removed from the fish and macroscopically assessed for sex, development, and condition and placed into a barcoded vial containing 10 percent formalin to preserve cell structures and prevent tissue degradation;
- Preserved gonads are microscopically assessed for sex, oocyte diameter, development, and batch fecundity estimates;
- Grossing process to remove a cross section of the gonad for histological preparation;
- Embedded gonad histologically assessed for sex, prior spawning indicators, atresia, post ovulatory follicles, and reproductive phases.
Preserved female samples collected during peak reproductive season can also be utilized to obtain eggs counts and batch fecundity estimates. Counting fish ova and extrapolating results to the population level is very labor intensive. Often a simple proxy for fecundity is just the weight of mature fish (or females) since body size and body weight correlates with the size of the ovary and the number of ova it can contain, at least in cold water fishes with a determinate fecundity pattern. This approximation is referred to as spawning stock biomass (SSB) when all the weights of fish (or females) in an age class are averaged together. But the rule about reproductive output scaling with body size to maximize number of eggs breaks down in the tropics where females instead spawn throughout much of the year. Females are often smaller in the tropics than their related counterparts in cold waters but that doesn’t mean they can’t have as great or even greater reproductive output. So the simple body mass correlation doesn’t work as well. Complicating matters, recent work questions some common assumptions about fish reproduction, such as whether mature fish typically spawn every year or whether fish can undergo smaller scale fine tuning/ regulation at a point closer to spawning. Instead of merely managing for levels of spawning stock biomass, the studies we conduct as well as other recent investigations are finding that having an older or more diverse age structure is also an important management goal for maximizing reproductive success.
Histology and Tissue Examination
We routinely examine samples macroscopically (unassisted) and microscopically (under a dissecting microscope). In order to obtain additional information regarding gamete development, it is necessary to assess the samples histologically. Histological examinations provide information necessary to generate reproductive parameters to assist with stock assessment models. The reproductive samples selected for histology are preserved in 10 percent buffered formalin, trimmed and grossed in-house. The trimmed tissue is then sent off in histology cassettes to a laboratory for histological processing and examined at the Panama City Laboratory.