1. Ovarian development and maturity of Southern Flounder in the north-central Gulf of Mexico
Morgan M. Corey1, Nancy J. Brown-Peterson2, Robert T. Leaf1, Samuel D. Clardy3, & Mark S. Peterson1
1Department of Coastal Sciences, The University of Southern Mississippi, Ocean Springs, MS 39564
2Center for Fisheries Research and Development, The University of Southern Mississippi, Ocean Springs, MS 39564
3Marine Education Center, The University of Southern Mississippi, Ocean Springs, MS 39564
Introduction
Results
Conclusions
The reproductive biology of Southern Flounder is poorly described in the Gulf of Mexico (GOM).
Spawning occurs following an offshore migration in winter1.
Descriptions of maturity and spawning dynamics are needed to inform estimates of spawning stock biomass.
Females first reached maturity at 245 mm TL and all were mature by 368 mm TL.
Estimated L50 was 304 mm TL (Figure 2) with 95% confidence interval 296 to 311 mm TL.
Age50 was estimated at 1 year.
L50 was estimated as 304 mm TL, about equal to the 12-inch minimum length limit in Mississippi.
Female-specific L50 estimate is significantly lower than previously reported estimates in the Atlantic3 but significantly higher than L50 estimates reported in the GOM1.
Age50 is in agreement with estimates from past studies3.
Both GSI and histological data indicate that the reproductive season duration is from November to February, but no actively spawning females were collected.
Results from histological analysis indicate that Southern Flounder exhibit batch spawning with asynchronous oocyte development.
Objectives
Figure 4.
Monthly percent reproductive phase for female Southern Flounder (n = 332). Phases: immature (Imm), early developing (EDev), developing (Dev), spawning capable (SC), regressing (Reg), and regenerating (Rgn).
Estimate mean length- and age-at-50% maturity for females
Define spawning seasonality
Describe ovarian development
Immature ovaries were identified by packed primary growth oocytes with interstitial tissue (Figure 5a).
The transition from developing to spawning capable was marked by an increase in later vitellogenic stage oocytes (Figure 5b,c).
Spawning capable females exhibited asynchronous oocyte development (Figure 5c).
Regressing ovaries had atresia present (Figure 5d).
Materials and methods
Figure 2.
Logistic model describing the length-at-50% maturity (L50) for female Southern Flounder (n = 332).
Understanding Southern Flounder reproductive biology can inform stock assessment and improve state-level management.
Fish were collected in the north-central GOM (Figure 1), and measured for total length (TL, mm) and body weight (g).
Gonads were removed, weighed (g), and preserved for histological processing.
Reproductive phase was classified using histological indicators to assign maturity2.
Length-at-50% maturity (L50) and age-at-50% maturity (Age50) were estimated using a logistic model.
Mean monthly gonadosomatic index (GSI) was calculated with standard error.
Mean GSI increased from October to November with a maximum in November (Figure 3).
Figure 5.
Histological images for females in the immature (a), developing (b), spawning capable (c), and regressing (d) phases. Oocyte stages: primary growth (PG), cortical aveolar (CA), primary (Vtg1), secondary (Vtg2), and tertiary vitellogenic (Vtg3). a c d b PG CA
Vtg2
Vtg1
atresia
interstitial tissue
Vtg3
References
1Fischer Unpublished MS Thesis, LSU, Baton Rouge, LA. 68 p.
2Brown-Peterson et al Marine and Coastal Fisheries 3(1):
3Midway and Scharf Marine and Coastal Fisheries 4(1):
Figure 3.
Monthly GSI (mean ± SE) for sexually mature female Southern Flounder (n = 277).
Acknowledgments
Spawning capable females were observed in November and December, and regressing females were observed in January and February (Figure 4).
No actively spawning females were collected.
Funding for this research provided by the Mississippi Tidelands Trust Fund Program. We thank those who assisted in sample collection.
Figure 1.
Map of locations where Southern Flounder (n = 369) were collected using gigs, hook and line, and trawls.