This is the third snook stock assessment since the slot-size limit was implemented in 1999.
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The 2012 Stock Assessment Update of Common
Snook, Centropomus undecimalis
Robert G. Muller and Ronald G. Taylor
Florida Fish and Wildlife Conservation Commission
Florida Fish & Wildlife Research Institute
100 Eighth Street, Southeast
St. Petersburg, Florida 33701-5020
May 30, 2012
- This assessment integrated life history studies and fishery data from a variety of sources including Fish and Wildlife Conservation Commission (FWC) Fish and Wildlife Research Institute’s (FWRI) snook project, angler-supplied snook carcass and logbook data, the National Marine Fisheries Service’s Marine Recreational Fishery Statistics Survey (MRFSS), and the Everglades National Park’s (ENP) recreational creel surveys into age-based assessments of snook populations on Florida’s Atlantic and gulf coasts. The most comparable data came from the years 1986 through 2010, but information from 1981 to 1985 was also presented.
- Commercial snook landings from 1918 until 1957, when the Legislature declared snook a gamefish, were presented to provide a historical context. Statewide recreational landings expressed in pounds in recent years equal or exceed the earlier commercial landings.
- Because of a severe cold spell in January 2010, the opening of the snook fishery in February was cancelled on both coasts in 2010 and 2011. The fall fishery (September – November) was opened on the Atlantic coast in both years but not on the on the gulf. Thus, the snook fishery was primarily a catch-and-release fishery for most of 2010 and 2011.
- In 2010, based on MRFSS interviews with anglers who indicated the type of fish they were targeting, snook dropped to the seventh most targeted species on the Atlantic coast (down from fifth place in 2009) and eleventh most targeted on the gulf coast (down from third place in 2009). Several measures of effort, such as the estimated number of directed snook-fishing trips, species preference, and sales of snook stamps, indicated that the fishing effort directed at snook was high, although lower in 2010, even though snook only occur in southern Florida.
- Catch rates for MRFSS and Everglades National Park were calculated with generalized linear models. The terms that significantly reduced the deviance in the MRFSS catch rates by coast included year, two-month wave, region, time fished, mode, and avidity and the terms included for the Everglades National Park catch rates were year, month, time fished, area fished, and launch site.
- On the Atlantic coast, MRFSS total-catch rates have been flat since 2001 and the catch rates on the gulf coast generally have been increasing. On both coasts in 2010, the catch rates dropped off sharply.
- Total catch rates from the Everglades National Park creel survey, which were applied to the gulf coast analyses, showed similar increases in recent years as did the MRFSS index and also experienced the sharp drop in 2010.
- The fishery-independent haul seine catch rates in numbers of snook per set provided by the FWC’s Fishery Independent Monitoring program decreased over the 1997-2010 time series on the Atlantic coast with 2010 having the lowest catch rate while catch rates on the gulf coast from 1996 to 2010 have generally increased reaching a peak in 2008 and then dropping in 2010 to the lowest catch rate in the series. We used age-length keys by coast from the fishery independent samples to prorate the indices to generate an index for age-2 fish.
- The sizes of fish that anglers released were estimated from angler interviews, logbooks and the Snook and Gamefish Foundation’s Angler Action Program. Data from the first two programs were available beginning in 2002 and the Angler Action Program for 2010. We used the average lengths from 2002-2005 by coast to assign lengths to the released fish in the years prior to 2002.
- This assessment only used the National Marine Fisheries Service’s Age-Structured Assessment Program (ASAP) because ASAP is more flexible than the previously used Integrated Catch-at-Age model especially in terms of numbers of indices, years with age data for estimating selectivity, linking discards to their fisheries, differential weighting of population parameters, and other technical details.
- Fishing mortality rates on the Atlantic coast tracked those estimated in earlier assessments until 1998 and then the rates were higher for 1999-2003 and similar in 2004. Not surprisingly given the extended season closure, the 2010 estimate of average fishing mortality for the reference age (Age-5) on the Atlantic coast was low at 0.04 per year but it must be noted that fishing mortality rates have been declining on the Atlantic coast after 2000. On the gulf coast, the fishing mortality rates declined after the slot limit was implemented in 1999 and in 2010, the fishing mortality for the same reference age was 0.04 per year down from the 1997 high value of 0.53 per year. Again it must be noted that the fishery on the gulf coast was closed for all of 2010 in response to the cold kills in January 2010.
- The spawning biomass of snook decreased on the Atlantic coast until 2001 and then has slowly increased while the spawning biomass on the gulf coast has been stable since 2003. A model run for the gulf coast that incorporated the cold kills in 2001, 2003, and 2010 as well as red tides in 2003, 2005, and 2006 estimated a 22% drop in spawning biomass in 2010.
- The estimated recruitment on the gulf coast from 2008 through 2010 was the lowest in the time series and these low numbers of recruits occurred at the highest spawning biomass values. This combination of low recruitment stemming from high spawning biomass indicates that the environmental effects on recruitment are important and should be investigated.
- In July 2007, the Commission implemented the Snook Work Group’s recommendations to adopt a one-fish bag limit statewide and to narrow the slot limits to 28-32 inches total length (TL) on the Atlantic coast and 28-33 inches TL on the gulf coast. Based upon the average lengths of kept fish recorded by both MRFSS and ENP samplers for the 2008-2010 time period, anglers on the Atlantic coast had reasonable compliance (71% of the kept fish) within the 28–32 in slot limit and anglers on the gulf coast had less compliance (55% of the kept fish) within the 28-33 in slot limit. On the Atlantic coast 12% of the fish were undersized (less than 28 in) and 16% were oversized (greater than 32 in) while on the gulf coast 42% of the fish caught were undersized and only 3% were oversized (greater than 33 inches). During 2008–2010, only two out of 1,552 anglers intercepted on the Atlantic coast (one out of 791 trips) kept more than one fish per angler while four out of 4,562 anglers intercepted on the gulf coast (two out of 2,075 trips) kept more than one fish per angler.
- The combination of the one-fish bag limit and the narrower slot limit has reduced the recreational fishery catchability by 40% on the Atlantic coast and 71% on the gulf coast when compared to the catchability prior to 1999.
- The Commission’s management objective for snook is to maintain the spawning potential ratio (SPR) at or above 40%. The transitional SPR (tSPR) values in 2010 exceeded the Commission’s objective on the gulf coast (51%) and approached the goal on the Atlantic coast (36%). The model run for the gulf coast that included cold kills and red tides estimated a tSPR of 56%. However, there are limitations to using SPR as the only metric of stock status. SPR was devised to evaluate the effect of fishing mortality on a stock. Natural mortality events, such as red tides or cold kills are not accounted for in SPR model results. It is recommended that management of stocks like snook that are more vulnerable to episodic environmental events such as red tides and cold kills should be based on other benchmarks or a combination of benchmarks, such as SPR and a target level of SSB.