Frontal Periods 



A frontal period was arbitrarily defined as any 

 four consecutive days the first of which the water 

 temperature dropped 2°C or more. Four days 

 were selected, because fish catches were gener- 

 ally affected for 2 to 4 days following the initial 

 temperature drop on the first day of a frontal 

 period. Six frontal periods occurred in the study 

 area from 17 September to 13 December (Figure 

 2). Fronts moved through the study area on 17 

 October, 28 October, 9 November, 27 November, 5 

 December, and 10 December (Figure 2). The av- 

 erage decrease of water and air temperatures per 

 24-h period for the above dates was 2.5°C and 

 6.4°C, respectively. In addition to decreases of 

 temperatures, fronts passing through estuaries of 

 the northern Gulf of Mexico are also charac- 

 terized by: 1) rapid changes in barometric pres- 

 sure, 2) shifts in wind direction and wind speed, 

 3) changes in tidal heights, and 4) increases in 

 turbidity and velocity of tidal currents (E. J. Pul- 

 len, pers. commun., U.S. Corps of Engineers, Gal- 

 veston, Tex.). 



Catch Related to Frontal Periods 



Spanish mackerel (Table 2, Figure 3) were the 

 exceptions. Spanish mackerel was the only 

 species caught in greatest numbers between fron- 

 tal periods. Mean catches of the nine species 

 ranged from 1.7 to 9.5 times greater during fron- 

 tal periods than between frontal periods. 



Mean lengths of fish caught during frontal 

 periods were not significantly different from 

 those caught between frontal periods for each of 

 the 10 most abundant species (Table 2). 



These results suggest that many species of 

 marine fishes become more vulnerable to capture 

 by gill nets in shallow areas of coastal bays dur- 

 ing frontal periods in autumn. This increased 

 vulnerability probably results from increased ac- 

 tivity, migration, a lessening ability to avoid the 

 net, and one or more of the factors associated with 

 fronts, e.g., changes in temperature, tidal height, 

 turbidity, and current velocity. 



Acknowledgments 



Our sincere appreciation is extended to J. R. 

 Lara for fiarnishing climatological data and to D. 

 B. Jester, M. A. Roessler, and J. Y. Christmas for 

 their helpful comments. 



Each front was characterized by a marked in- 

 crease in the numbers of individuals caught. Such 

 a marked increase occurred only once (22-24 

 November) during a nonfrontal period (Figure 2). 

 The mean number (all species combined) of fish 

 caught per day was 354.7 during frontal periods 

 and 113.1 between frontal periods (Table 2). Mean 

 catches were significantly higher during frontal 

 periods for all species combined and for 8 of the 10 

 most abundant species. Atlantic croaker and 



Literature Cited 



Hopkins, T. L. 



1966. The plankton of the St. Andrew Bay system, Florida. 

 Publ. Inst. Mar. Sci., Univ. Tex. 11:12-64. 



ICHIYE, T., AND M. L. Jones. 



1961. On the hydrography of the St. Andrew Bay system, 

 Florida. Limnol. Oceanogr. 6:302-311. 



Steel, R. G. D., and J. H. Torrie. 



I960. Principles and procedures of statistics with special 

 reference to biological sciences. McGraw-Hill, N.Y., 

 481 p. 



Table 2.— Comparisons of mean catches per day and mean lengths during and between frontal periods, September-December 



1973, St. Andrew Bay, Fla. 



452 



