1. When is Spawning Biomass a Good Proxy for Stock Reproductive Potential?
A Case Study of Red Snapper (Lutjanus campechanus) in the Gulf of Mexico
SEFSC
Clay E. Porch and Gary. R. Fitzhugh
American Fisheries Society
Portland, Oregon
August 2015
Illustration: Diane Rome Peebles

2. Impetus
Stock assessments often use mature biomass to measure reproductive potential
But…egg production is seldom proportional to somatic growth
Weight = aL3
Eggs = bL4-5

3. Gulf of Mexico Red Snapper are like that
Batch fecundity ( = aL 4.2 )

4. Gulf of Mexico Red Snapper also spawn more often with age
Porch et al Marine and Coastal Fisheries.

5. many fish are immature during the period of fast somatic growth
after maturity, gains in egg production faster than gains in weight

6. Per capita reproductive potential of red snapper measured in weight and eggs

7. Total reproductive potential of red snapper measured in weight and eggs (at MSY)

8. So what? THE RED SNAPPER POPULATION IS THIS BIG SPORT FISHERMEN
FISHERIES
SCIENTISTS

9. Trends in spawning indicators over time

10. Trends in spawning indicators over time
*Rescaled to mean

11. Spawning potential equilibrium reference points

12. Perceived spawner-recruit relationship

13. Example 1: True h = 0.7 additive error

14. Example 2: True h = 0.7 additive error No observations at large stock sizes

15. Example 3: True h = 0.7 multiplicative error

16. Conclusions w.r.t. stock assessments
Mature number is poor proxy for red snapper spawning
Mature biomass may not be too bad for fish like red snapper
Underestimates steepness (= fish less)
Over-estimates contribution of young fish (= fish more)
SPR reference points overestimated by up to 40% (for low targets)
There may be other important age/size dependent factors like
egg quality
spawning locations

17. Conclusions w.r.t. stock assessments
Mature number is poor proxy for red snapper spawning
Mature biomass may not be far off for fish like red snapper
Underestimates steepness (= fish less)
Over-estimates contribution of young fish (= fish more)
SPR reference points overestimated by up to 40% (for low targets)
From a stock assessment perspective, for species where reproductive capacity is unknown, first priority should be placed on determining maturity (which is cheap anyway)
But still important to pursue quantifying egg production, especially for species where reference points are proposed with corresponding SPR’s below 40%

18. Pensacola red snapper fishery
Questions?
Pensacola red snapper fishery
1940s
Photo credit: Pensacola News Journal

19. Stock-recruitment relationship