Red Porgy Reproductive Analysis

Protogynous fishes are often assumed to be more sensitive than gonochorists to exploitation, primarily because of potential sperm limitation and/or social disruption of mating if larger, mostly male individuals are selected for.

Almost 4000 red porgy were collected year-round, Mar 1998-Sep 2001, in the NE Gulf, primarily using standardized hook and line gear in depths of 22-92 m. Most (n=2,586) were collected at 9 sites off NW Florida in 30-68 m sampled repeatedly. Fish were aged using whole and broken and burned sagittae, and sexed and staged histologically (females and transitionals)or macroscopically (males). An early objective was to determine if the behaviorally-related size and/or sex selectivity observed in other protogynous species occurred in red porgy, i.e., were larger individuals or males at a site more aggressive and more likely to bite a hook and be caught before smaller ones or females. Non-parametric runs tests of ordered size and sex data showed no evidence of such selectivity, indicating that hook-and-line gear is a fair way to sample red porgy. More importantly, any evidence of truncation in size structure or skewing of sex ratios in exploited populations should not be attributed to greater aggression or '"hook attraction" in males but can be easily explained as the results of simple size-selective harvesting.

Histological evidence indicated that red porgy in depths of 20 to 78 m in the NE Gulf spawn wherever they occur, primarily Dec to Feb. Estimated size and age at 50% maturity for females was 211 - 216 mm TL and <2 yr. Sex change occurred wherever they were found, almost exclusively Mar-Nov, and across a wide range of sizes (206-417 mm TL) and ages (2-9 yr), strong evidence it is socially controlled. Red porgy are permanently sexually dichromatic. The premaxilla is green or bluish-green in males and pink or reddish in females. Observations of captive fish suggest they pair spawn. Seasonal patterns in catch rates and sex ratios, and widespread occurrence of spawning females indicated that red porgy do not form large, predictable spawning aggregations.

There was no evidence found that protogyny or their reproductive ecology might 1) explain the apparent crash of the red porgy stock(s) in the SAB or 2) make the species more sensitive to exploitation than gonochorists - in fact they are probably less sensitive in some cases. Many aspects of their biology and behavior, including widespread spawning grounds, no tendency to form spawning aggregations, absence of behaviorally-related size or sex selectivity, socially controlled sex change, co-occurrence of sexes year-round, and an extended period of transition, should stabilize or enable rapid compensation of sex ratios (preventing sperm limitation or disruption of mating). Socially controlled sex change also enables size and age of transition to slide downward as fishing truncates the size structure, similar to the declines in size and age at maturity seen in many gonochorists. Several population traits differed significantly among the 9 regular sites (which ranged from 1.3 to 58.4 km (0 = 28.7) apart), including size and age composition (K-S 2 sample test);

means ranged from 261 to 309 mm TL and 2.7 to 4.1 yr. Size at age varied considerably, primarily because of significant differences among sites. The relationships of mean size at age among sites were consistent across ages and temporally stable. Robson-Chapman maximum likelihood estimates of annual survival ranged from 38 to 65 % among sites, and 95% CI?s did not overlap for 6 of the 8 sites with estimates. Logistic regression indicated that the proportion of females changing sex differed significantly among sites (medians: 12 - 33%) and depths.

Sizes and ages at transition also varied spatially, with site-specific means of 266 - 313 mm and 3.1 - 4.6 yr. Sex ratio was yet another demographic that differed among sites: 28 of 36 pairwise comparisons were significant (log. regress.).

These persistent differences in population traits at such a small scale likely reflect phenotypic, not genetic, effects. Two factors - spatial heterogeneity of their environment and site fidelity - probably explain most of those differences. The live bottom habitat preferred by red porgy is widespread but very patchy. These patches, grossly similar, have variable hydrological, geological, biological, and ecological characteristics; and they range from unexploited to heavily exploited. Biological and ecological characteristics likely to vary among patches include density, predator and prey composition and density, and competition. Adult redporgy exhibit considerable site fidelity, so once recruited to a given patch of habitat, they are exposed to a unique suite of many factors which could affect growth, mortality, and reproduction. The consistent, persistent, significant differences in size and age structure, growth, xvi mortality, transition rates, size and ages at transition, and sex ratios among sites separated by only 10?s of kilometers strongly suggests that red porgy in the NE Gulf have a complex population structure composed of many local subpopulations. These subpopulations closely resemble Crowder et al. (2000) definition of sources and sinks areas of differing demographic rates dictated by underlying differences in habitat quality?. This complex structure is not the classical metapopulation of Levins (1970), i.e.,a A population of populations that go extinct and recolonize @and which are exposed to the same conditions in each habitat patch. It does, however, fit the broader definition of metapopulation espoused by Hastings and Harrison (1994), Hanski and Simberloff (1997), and Kritzer and Sale (2004), which relaxes the requirement for extinctions and recolonizations and does not require uniform conditions across patches.

Whether the population subunits are called local subpopulations, sources and sinks, or members of a metapopulation, the critical point is that many may have significantly different demographics and life history traits, which has potentially significant implications regarding stock assessment and management of red porgy. Data pooled from several subpopulations may yield skewed parameter estimates, which in turn could bias stock assessments and the models used to predict responses to exploitation. It could also introduce excessive variability to the parameter estimates. Such complexity could certainly frustrate and confound the efforts of those trying to assess the status of these stocks and predict the effects of fishing on them, as it requires examination of population biology at much smaller spatial scales than typically done and use of more complex, spatially-explicit population models.

It is likely that small scale population complexity has played some part in the failure of some southeastern U.S. reef fish fisheries to respond to management measures in recent years

Tags: 1998-2001, age structure, alabama, animals/vertebrates, atlantic ocean, biological classification, demography, doc/noaa/nmfs/sefsc, earth science, fish, fish species, florida, growth, gulf of america, gulf of mexico, life history, mortality, national marine fisheries service, noaa, north atlantic ocean, ocean, otolith, pagrus pagrus, protogyny, red porgy, red porgy study, reproductive ecology, southeast fisheries science center, species identification - age, species identification - count, species identification - sex, u.s. department of commerce
Organization: National Oceanic and Atmospheric Administration, Department of Commerce