Reproduction Dynamics

At the Fish and Wildlife Research Institute (FWRI), scientists are studying where and when reef fish spawn in the Gulf of Mexico to better understand the level of fishing pressure these economically important stocks can sustain.

Although incorporating detailed reproductive data into all stock assessments is not practical, researchers are increasingly recognizing the need to understand factors driving population productivity and the role reproductive biology plays in this productivity. Given that a species’ reproductive compensatory ability depends on the selection pressures under which it evolved, an understanding of reproductive traits – genetic basis and phenotypic plasticity – is important to better understand population productivity and resilience to fishing pressure. Such an understanding will improve the FWC’s ability to manage fish stocks for sustainability. 

Reproductive success – the production of offspring that live to reproductive age – depends on reproductive output and offspring survival. Because of this, reproductive studies focus on estimating fecundity and, to a lesser degree, understanding adult traits that impact offspring survival, such as where and when fish spawn.

To understand these reproductive traits, biologists from FWRI’s Marine Fisheries Research section in 2008 began evaluating reproductive samples from reef fish collected for the research and monitoring program. As of 2011, researchers had samples from more than 1,000 fish.

Researchers’ long-term goal is to use this data to develop baseline knowledge of where and when key species spawn and use this foundation to develop species-specific reproductive studies to estimate fecundity and reproductive behavior. The following are objectives for the first stage:

• Developing a photographic reference guide to reef fish histology, with examples of key oocyte developmental stages and reproductive phases 
• Mapping the spawning sites of target species by assessing where actively-spawning females were collected
• Assessing reproductive timing of these species by evaluating the season and time at which females of different reproductive phases – e.g., developing, spawning capable, actively spawning and regressing/regenerating – are collected (terms used for phases from Brown-Peterson et al. 2011)

Researchers collect reproductive data using a range of sampling techniques, including egg and larval surveys, visual observation and tagging. The basis of most reproductive data, however, is a measure of gonadal development, which is most accurately assessed with gonadal histology. Histological analysis assesses the most developed stage of gametogenesis (gamete development), as well as several other key indicators, such as post-ovulatory follicles and atresia.

Similar to recent efforts to review, standardize and disseminate advances in otolith research and its application to fishery science, researchers are also working to standardize reproductive terms. FWRI researchers aim to advance this effort by following the terminology presented in Lowerre-Barbieri et al. 2011 and Brown-Peterson et al. 2011.

Most exploited marine teleosts are highly fecund and produce either pelagic or demersal eggs. All fish undergo sexual maturation, participate in one or more reproductive cycles, spawn once or more per cycle, age and die. Reproductive timing, however, can range from spawning only once during a lifetime – as does the coho salmon (Oncorhynchus kisutch) – to spawning multiple times during an extended spawning season for many years, which is a common pattern among reef fish, such as red snapper (Lutjanus campechanus). 

To determine reproductive timing patterns in reef fish, FWRI researchers evaluate the stage of gametogenesis fish are undergoing at the time of capture. Gametogenesis reflects whether or not a fish has reached sexual maturity, where it is within the spawning cycle and its proximity to spawning. Gametogenesis is similar for all teleosts, with spermatogenesis (the development and growth of sperm) proceeding through universal stages of development: spermatogonia, spermatocytes, spermatids and spermatozoa. Oogenesis (the development and growth of oocytes) is also similar for all fish and typically shows the following progression:

• Oogonia (Oo)
• Primary growth (PG) oocytes
• Previtellogenic stage in which oocytes increase in size and often acquire oil droplets and cortical alveolar (CA) vesicles
• Largely estradiol-driven vitellogenic stage (Vtg)
• Oocyte maturation (OM) and ovulation

The progression of oocyte development begins with primary growth oocytes (PG), which is the most advanced stage of oocyte development in immature fish. Secondary growth refers to the oocyte developmental stages seen only in mature or maturing females and begins with a previtellogenic stage commonly called cortical alveolar (CA). During this stage oocytes increase in size; develop a well-defined zona radiata; and typically acquire lipid droplets, cortical alveoli or both. There are important applications associated with identifying the CA stage – which indicates commitment to a given reproductive cycle – such as determining fecundity type (indeterminate or determinate) and assessing maturity.

Vitellogenesis, which can be broken into substages, is associated with the extent of yolk globules, or platelets, in the ooplasm (Vtg1-Vtg3). Oocyte maturation (OM) is associated with the resumption of meiosis, indicating the fish is preparing to spawn. The progression of OM and, thus, proximity to spawning can be assessed based on the following substages: germinal vesicle migration (GVM), yolk coalescence (YC), germinal vesicle breakdown (GVBD) and, in pelagic spawners, hydration (H).

At ovulation, the follicle ruptures and the oocyte is released. Most fish spawn immediately after ovulation. Postovulatory follicles (POF) remain in the ovary, where they are resorbed.

Researchers use histological analysis of gonad tissue to determine reproductive state – immature or mature, and for mature females, spawning or nonspawning – and reproductive phase. This information is then used in conjunction with capture date, time and location to estimate the spawning season, time of spawning and spawning sites – sampling sites with actively spawning fish. Using samples from this study, researchers documented red snapper spawning activity in the Gulf of Mexico west of Tampa Bay, an area not previously known as spawning habitat for this important species.

Conserving the spawning population of a stock is a fundamental goal of marine fisheries management, but researchers often do not know where and when fish spawn. By linking reproductive analysis with the large-scale sampling done by FWRI’s Fisheries-Independent Monitoring group, biologists are beginning to better understand reef fish reproductive dynamics. This understanding, in turn, will help fisheries managers develop strategies to protect spawning populations.