684 



Abstract-The stock status of the 

 Gulf of Mexico migratory group of king 

 mackerel (Scomberomorus cavalla) is 

 currently evaluated by using age- 

 based sequential virtual population 

 analysis (VPA). We examined king 

 mackerel larval occurrence and abun- 

 dance indices from annua! ichthyo- 

 plankton surveys, developed an age- 

 adjusted abundance index, and ques- 

 tioned whether larval indices of abun- 

 dance are useful for calibrating the 

 king mackerel VPA. Gulf of Mexico 

 king mackerel larval occurrence and 

 abundance increased over a fourteen- 

 year time series from 1982 to 1995 

 and were highly correlated with spawn- 

 ing stock size. Correlations between 

 stock size and larval occurrence, and 

 between stock size and larval abun- 

 dance, were 0.82 and 0.84, respectively. 

 The correlation between larval occur- 

 rence and stock size for the years 

 1986-95 increased to 0.91, owing to the 

 addition in 1986 of a fall survey with 

 added coverage during peak spawning. 

 Daily instantaneous mortality rate (Z) 

 was estimated by regression of larval 

 catch cui-ves. Although a large amount 

 of interannual variability in mortality 

 rates was noted, no statistical differ- 

 ences were detected among years. The 

 instantaneous daily mortality rate esti- 

 mated by pooling all years, Z = 0.53, 

 was used to develop an age-adjusted 

 index for king mackerel in order to elim- 

 inate the influence of variable lai-val 

 age composition among years. This 

 adjusted index did not improve corre- 

 lations between stock size and larval 

 abundance (;-=0.78). For now, indices 

 of larval occurrence and unadjusted 

 larval abundance from ichthyoplankton 

 collections reflect trends in spawning 

 stock size and provide useful variables 

 for calibrating the king mackerel VPA. 



Indices of larval king mackerel 

 {Scomberomorus cavalla) abundance in the 

 Gulf of Mexico for use in population assessments 



Christopher T. Gledhill 



Joanne Lyczkowski-Shultz 



Southeast Fisheries Science Center 



National Marine Fisheries SeiA/ice, NOAA 



3209 Frederic St 



Pascagoula, Mississippi 39568-1207 



E mail address (for C T Gledhill): ChristopherTGIedhill gnoaa gov 



Manuscript accepted 4 April 2000. 

 Fish. Bull. 98:684-691 (2000). 



The king mackerel I Scomberomorus ca- 

 valla ), a western Atlantic member of the 

 family Scombridae, ranges from Mas- 

 sachusetts to Brazil (Shipp, 1986). This 

 highly migratory, coastal pelagic spe- 

 cies can attain a maximum size of 1.7 m 

 and 45 kg (Robins and Ray, 1986) and 

 ages of more than 20 years (DeVries 

 and Grimes, 1997). King mackerel sup- 

 port valuable commercial and recre- 

 ational fisheries that are regulated in 

 the southeastern coastal states and 

 Gulf of Mexico under the Coastal Migra- 

 tory Pelagic Resources Fishery Man- 

 agement Plan. These fisheries were 

 largely unregulated in the late 1970s 

 and early 1980s when fishing mortal- 

 ity was high, and thus stock size was 

 reduced. As a result, management by 

 quota was implemented in the 1985-86 

 fishing year. The current management 

 regime for king mackerel fisheries rec- 

 ognizes only two stocks off the U.S. 

 southeast coast: an Atlantic migratory 

 group and a Gulf of Mexico migi'atory 

 group. There is, however, some dis- 

 agreement as to whether there are one 

 or two distinct stocks in the Gulf of 

 Mexico (Johnson et al., 1994; DeVries 

 and Grimes, 1997; Gold et al., 1997; 

 Roelke and Cifuentes, 1997). 



Reproduction in tliis liighly fecund, 

 serial spawning species occurs from May 

 through early October and peaks in Sep- 

 tember along both the U.S. Gulf of Mexico 

 and southeast Atlantic coasts (McEachran 

 et al., 1980; Finucane et al., 1986; Grimes 

 et al., 1990). Data on the abundance and 

 distribution of king mackerel larvae indi- 

 cate that spawning occurs chiefly over 

 the mid to outer continental shelf of the 

 northern Gulf of Mexico (McEachran et 



al, 1980; Grimes et al., 1990). Grimes 

 et al. (1990) suggested that spawning 

 occurs over shallower depths in the region 

 from west Louisiana to northwest Flor- 

 ida and may be associated with oceano- 

 graphic features, especially the discharge 

 plume of the Mississippi River Absolute 

 growth rates of king mackerel larvae 

 were observed to range from 0.54 to 1.33 

 mm per day and were slightly higher 

 for larvae from the Mississippi River 

 plume when compared to other locations 

 in the Gulf and southeast Atlantic coast 

 (DeVries et al, 1990). 



Population size of Gulf-group king 

 mackerel is estimated biennially by sci- 

 entists of the Mackerel Stock Assess- 

 ment Panel (Gulf of Mexico and South 

 Atlantic Fishery Management Coun- 

 cils) using an age-based sequential vir- 

 tual population analysis (VPA). This 

 VPA is implemented by using ADAPT 

 (Conser and Powers, 1990; Powers and 

 Restrepo, 1993 ). The king mackerel VPA 

 is calibrated or tuned by using various 

 indices of abundance based on fisheries 

 dependent catch-per-unit-of-effort and 

 fisheries independent resource sui^vey 

 data. Survey estimates of annual abun- 

 dance and frequency of occurrence of 

 king mackerel larvae were first consid- 

 ered as tuning variables for the king 

 mackerel VPA by the 1996 Mackerel 

 Stock Assessment Panel. ' Although fre- 



Mackerel Stock Assessment Panel. 1996. 

 1996 Report of the Mackerel Stock Assess- 

 ment Panel. Gulf of Mexico Fishery Man- 

 agement Council, Lincoln Center, Suite 

 .331, 5401 West Kennedy Blvd. Tampa, FL 

 33609 and South Atlantic Fishery Manage- 

 ment Council, Southpai-k Bldg., Suite 306, 

 1 Southpark Circle, Charleston, SC 29407. 



