48 



Fishery/ Bulletin 100(1) 



in Florida and 131-200 mm in Mexico. We found a maxi- 

 mum age of seven yeai's. vvherea.s two years was suggest- 

 ed by Martinez-Palacios and Ross (1992). We offer three 

 explanations for these observed differences in the length- 

 at-age data. First, exploitation rates may differ between 



250 



Figure 5 



Obsen'ed and predicted lengths at age for male and female Mayan 

 cichlids from the von Bertalanffy gi'owth model. /)=obsei'ved and 

 back-calculated size-at-age information from 148 males and 157 

 females. 



study areas. Fish in our study came from the Crocodile 

 Sanctuary within Everglades National Park and had not 

 been exposed to fishing mortality. Fishing for Mayan cich- 

 lids occurs outside of our study area, and heavy exploita- 

 tion can select for faster-growing fish with a shorter life- 

 span (Ricker, 1975 1. Martinez-Palacios and Ross 

 (1992) suggested that their population was over- 

 fished. Second, differences in temperature impact 

 fish growth. Colder winter temperatures in Florida 

 were sufficient to form seasonal marks in the oto- 

 liths of Mayan cichlids and may have caused slower 

 growth than in Mexican populations. Third, the sea- 

 sonal length frequencies of Martinez-Palacios and 

 Ross (1992) were insufficient to accurately identify 

 older year classes. Because growth slows with age, 

 the length-frequency of cohorts corresponding to 

 older age classes can overlap significantly, resulting 

 in erroneously lower age estimates. Future efforts 

 to age Mayan cichlids in Mexico should include 

 thin-sectioned otoliths to evaluate the findings of 

 Martinez-Palacios and Ross (1992). 



Although the Mayan cichlid has proliferated for 

 over a decade in the natural and man-made habitats 

 surrounding the Everglades, studies are only recent- 

 ly becoming available (e.g. Trexler et al., 2000 1. More 

 introduced fish species are found in Florida than 

 in any other state in the United States, and 13 of 

 the 18 species with established populations are cich- 

 lids (Shafland, 1996). The impact of exotic species 

 on native Florida fishes has been debated: Shafland 

 (1996) proposed no demonstrable effect on native 

 fishes, whereas Courtenay ( 1997 ) argued that lack 

 of available data precludes a determination. Trexler 

 et al. (2000) provided empirical data that support 

 Shafland (1996), concluding that although exotics 

 have been credited with native species extinctions 

 in other ecosystems, native Florida fishes are not 

 specialized or restricted to certain habitats and thus 

 are able to cope with the invasion of exotics. Finding 

 no drastic changes in the native ichthyofauna does 

 not necessarily mean that exotic species do not af- 

 fect indigenous fishes. Exotic species can introduce 



