2015 Common Snook Stock Assessment

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The 2015 stock assessment of common snook, Centropomus undecimalis

Robert G. Muller, Alexis A. Trotter, and Philip W. Stevens

Florida Fish and Wildlife Conservation Commission
Florida Fish and Wildlife Research Institute
100 8th Ave SE St. Petersburg, FL 33701

In House Report: IHR 2015-004

Executive Summary

  • The Commission closed the snook fishery by executive order after a severe cold spell in January 2010. The fishery on the Atlantic coast reopened in September 2010 and the fishery on the gulf coast remained closed until September 2013. An objective of this assessment, which includes data through 2014, was to evaluate the effects that the cold kill had on Common Snook populations.
  • The peak recreational harvest (those fish that were landed or died after being released alive) occurred on the Atlantic coast in 1997 (60,900 fish), averaged 32,000 fish from 1998 through 2004, and then declined to 6,500 fish harvested in 2010; the total harvest in 2014 on the Atlantic coast was 17,900 snook. The gulf harvest peaked at 149,000 fish in 1997 then declined before rising to 117,000 fish in 2004. Very low numbers of fish were harvested each year after 2009 with the closure of the gulf fishery. The fishery on the gulf coast reopened in September 2013 and the total harvest in 2014 on the gulf coast was 47,500 snook.
  • Common Snook is a popular recreational species even though it only occurs in peninsular Florida. In 2014, based on recreational interviews with anglers who indicated the type of fish that they were targeting, snook were the fourth most targeted species on the Atlantic coast and the third most targeted on the gulf coast. In the 2013 assessment, snook was the tenth most targeted species on both coasts.
  • Catch rates from the FWRI Fishery Independent Monitoring program (FIM), the federal Marine Recreational Information Program (MRFSS/MRIP) and the Everglades National Park creel survey (ENP) were used to provide guidance on stock trends to the assessment model.
  • The haul seine catch rates in numbers of snook per set from FWRI’s FIM program on the Atlantic coast decreased from 1997 to 2001, then stabilized until the drop in 2010, and then increased slightly. Catch rates on the gulf coast during 1996 to 2008 were mostly stable and then dropped in 2009 and 2010 to their lowest catch rates in the series. The gulf catch rates increased after 2010 with 2014 having the highest catch rate in the series. We used length data by coast for age-1 fish from the fishery independent samples to identify age-1 fish in the seine hauls and then repeated the standardization process to generate indices just for age-1 fish by coast. The catch rates for age-1 fish resembled the catch rates for adults.
  • On the Atlantic coast, recreational MRFSS/MRIP total-catch rates were flat from 1996 through 2009 then dropped in 2010 and 2011 before increasing to levels higher than the 1996-2009 levels. The catch rates on the gulf coast were generally increasing until a drop after the 2005 red tide event. The drop was followed by increases until the severe drop in 2010 and large increases since.
  • Total catch rates from the ENP showed similar increases in recent years as did the MRFSS/MRIP index on the gulf coast except with higher rates in 2007-2009. The ENP index showed the same sharp drop in 2010 followed by increases afterwards although only back to the levels seen in the early 2000s.
  • The sizes of fish that anglers released were estimated from angler interviews, angler logbooks, the Snook and Gamefish Foundation’s Angler Action Program, and a fishery independent catch-and-release fishing program. Data from the first two programs were available beginning in 2002 and the Angler Action Program beginning in 2010. The fishery independent catch and release fishing program operated from 2002 through 2005. We used the average lengths from 2002-2005 by coast from logbooks and the catch and release program to assign lengths to the released fish in the years 1999 to 2002. For the earlier years, we pro-rated these fish lengths after accounting for differences in size regulations.
  • As with the assessments since 2006, we determined the condition of the stocks using the National Marine Fisheries Service’s Age-Structured Assessment Program (ASAP) because ASAP is a flexible 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, estimating uncertainty, and other technical details.
  • Estimated fishing mortality rates on the Atlantic coast increased until 1998 (although the peak was 0.99 per year in 1995) and then the rates dropped with the implementation of more restrictive regulations (26-34 inch slot in 1999; one-fish bag limit and 28-32 slot limit in 2007). Fishing mortality rates were below 0.06 per year from 2010 until 2012 and then returned rates similar to those seen in 1999-2007. The fishing mortality rate in 2014 on the Atlantic coast was 0.16 per year. Fishing mortality rates also increased on the gulf coast before declining sharply after the min-max size limits were implemented in 1999 (the estimates of fishing mortality rate in 1996 was 1.48 per year and dropped to levels of around 0.20 per year after 1999). The fishing mortality rates during the closure attributed to the deaths of live-released fish were 0.03-0.04 per year during 2010-2012 and rose upon the fishery reopening. The fishing mortality rate in 2014 was 0.09 per year on the gulf coast.
  • The estimated recruitment on the Atlantic coast has generally declined since 1991-1992 and recruitment levels in 2007 and 2009 through 2011 were the lowest over the 29-year time period. Recruitment since 2012 has been higher at levels similar to those seen during the mid-1990s. Recruitment on the gulf coast has been mostly stable with a few years with high recruitment (2000, 2003, 2012, 2013, and 2014) and a few years with low recruitment (2005-2009). Low numbers of recruits on the gulf coast often occurred in years when there were high spawning biomass levels indicating that environmental effects were important determinants of year class strength.
  • The spawning biomass of snook on the Atlantic coast was generally flat until the late 1990s and then was flat at a lower level until 2011 when the biomass began to increase. While spawning biomass on the Atlantic coast was 345 mt (0.76 million lb) in 2014, one should expect lower spawning biomasses in the future when the fish from the low recruitment years (2009-2011) mature. The spawning biomass on the gulf coast has generally increased over the time series except for a decline beginning in 2008 that dropped to a low in 2010. The spawning biomass in 2014 (1,404 mt, 3.10 million lb) was the highest of the 29 years of data included in the assessment.
  • In July 2007, the Commission implemented the Snook Work Group’s recommendations to adopt a one-fish bag limit statewide and to reduce the min-max 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 recreational samplers for the 2008-2014 time period, anglers on the Atlantic coast had reasonable compliance with the new size limits: 79% of the kept fish were within the 28–32 in length limits, 4% were undersized, and 17% were oversized. Anglers on the gulf coast showed similar compliance with 73% of the kept fish within the 28-33 in length limits, 22% undersized and 5% oversized. Bag limit adherence was high on both coasts. During 2008–2014, only three out of 2,540 anglers intercepted on the Atlantic coast (two out of 1,609 trips) kept more than one fish per angler. On the gulf coast, twenty out of 21,052 anglers that were intercepted (12 out of 10,733 trips) kept more than one fish per angler during the period 2002-2014.
  • The Commission’s management objective for snook is to maintain the spawning potential ratio (SPR) at or above 40%. However, because 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. Therefore, results of this assessment are being presented both in terms of SPR as well as the equivalent spawning stock biomass (SSB). For the ‘base’ model (i.e., the model that does not incorporate the impact of environmental events), the transitional SPR (tSPR) values in 2014 approached the Commission’s objective on the Atlantic coast (39%) and exceeded the objective on the gulf coast (61%). Results from the model that incorporated the impact of environmental events (red tides and cold kills) indicated that in 2014 SSB in the Gulf was still at 90% of the SSB expected at SSB40%SPR (SSB2014/SSBSPR40%=0.90). Also, for the Atlantic SSB in 2014 was only at 60% of the SSB expected at SSB40%SPR (SSB2014/SSBSPR40%=0.60).




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