350 



Fishery Bulletin 98(2) 



the period of annulus formation (presumably in 

 winter) and the spawning season. It is distinctly 

 possible that otolith studies in these areas might 

 show significantly faster growth and younger fish. 

 This possibility is also raised by the rapid growth 

 rates of dolphinfish from these areas when main- 

 tained in aquaria (Herald, 1961; Beardsley, 1971; 

 Hassler and Hogarth, 1977), which are among the 

 highest reported (Table 3). 



The two-stock hypothesis of Oxenford and 

 Hunte ( 1986) predicts that fish sampled off Puerto 

 Rico from November to March (north coast fish) 

 should show distinctly different growth rates 

 from fish sampled from March to May (south 

 coast fish). However, no such differences were 

 found, and in general growth rates were similar 

 to those reported for the proposed southern stock 

 (Oxenford and Hunte, 1983). Figure 3 shows that 

 most fish sampled may have belonged to a single 

 cohort whose distribution shifted over time. The 

 size differences between fish from the north coast 

 (primarily January-February) and those from 

 the south coast (March-April) can be explained 

 by growth over the 8-month sampling period. 

 There is an influx of small fish in April off the 

 south coast; the growth rates of these fish are 

 equal to those of the rest of the sample. Although 

 these fish may represent the appearance of a 

 different (e.g. southern) stock, an equally likely 

 explanation is that they represent the entry of 

 a new cohort. Dolphinfish are known to spawn 

 several cohorts over the year (Beardsley, 1967; 

 Oxenford, 1985; Perez and Sadovy, 1996), and a 

 similar appearance of small fish occurs off Bar- 

 bados in June, where only one stock is hypoth- 

 esized. Annual length-frequency data from the 

 south coast of Puerto Rico (Fig. 4 in Perez and 

 Sadovy, 1996) show considerable variation in the 

 most abundant size class landed (from 800-1100 

 mm FL), which may reflect variations in the relative 

 strengths of the two cohorts. 



These observations do not necessarily negate the 

 two-stock hypothesis if, as postulated above, growth 

 rates for northern dolphinfish are much greater than 

 previously reported. In addition to growth rate, the 

 hypothesis is also based on differences in other life- 

 history parameters, but more recent studies cast 

 doubt on the significance of most of these. Data 

 in Perez et al.^ show trends in several parameters 



Fork length (mm) 



Figure 3 



Length-frequency distributions of dolphinfish from Puerto Rico 

 by month. Vertical position of the horizontal axes represents 

 the mean sampling date for each month. Curves indicate the 

 von Bertalanffy growth rate estimated from otoliths and are 

 positioned to indicate the main cohort offish sampled. 



Perez, R. N., A. Roman, and G. A. Rivera. 1992. Investigation 

 of the reproductive dynamics and preliminary evaluation of land- 

 ings data of the dolphinfish Cnryphaena hippurus, L. Puerto 

 Rico Department of Natural Resources, Puerto Rico Fisheries 

 Research Laboratory, P.O. Box 3665, Mayaguez, Puerto Rico 

 00680. Final Report D-J F26-1, 95 p. 



related to reproduction. Both the range and mean 

 size of mature oocytes show a gradual increase 

 in fish from Florida, Puerto Rico, and Barbados, 

 although sizes were identical for the north and south 

 coasts of Puerto Rico. A trend of increasing slope 

 among length-fecundity relationships was also evi- 

 dent.' Length of minimum maturity in females also 

 increased along the same gi'adient, from 324 SL mm 

 for Florida (Beardsley, 1967), 400 mm FL (369 mm 

 SL) for Puerto Rico,' to 610 mm SL for Barbados 

 (Oxenford and Hunte, 1986). These results, however, 

 could equally be interpreted as representing a cline, 

 as opposed to data for distinct stocks (Mahon and 

 Mahon, 1987). Genetic data showing differences is 

 based on the extremes of the distribution (Florida, 

 Barbados) and thus cannot be used to interpret what 

 occurs at a mid location such as Puerto Rico. 



