FISHERY BULLETIN: VOL. 8(;, NO. 4 



behind the waves (Table 2). The distribution of tar 

 balls (spilled asphalt) over and around the internal 

 waves was similar to the distribution of Sargassum 

 floats (Shanks 1987). As these internal waves prop- 

 agated shoreward, the currents over the waves 

 swept buoyant flotsam from the waters in front of 

 the set of waves into the convergence zone where 

 the flotsam was caught and carried shoreward. 



Calculation of the density of larvae or flotsam over 

 the internal waves (i.e., density in the slicks plus the 

 ripples) requires knowledge of the width of the slick 

 and rippled waters over the internal waves. Unfor- 

 tunately, in this initial study these measurements 

 were not made, necessitating that these widths be 

 approximated using values from the literature. 

 Slicks were assumed to be 30 m wide and the rip- 

 pled waters separating the slicks were assumed to 

 be 100 m wide. These values are consistent with my 

 experience and with published values (LaFond 1959; 

 Sawyer 1983). The observed density over one inter- 

 nal wave was calculated as (slick no./m^ x 30 m) -i- 

 (ripples no./m^ x 100 m)/130 m-. The density over 

 an internal wave was compared with the density in 

 an equal area of water in front of the set of inter- 

 nal waves. Making these calculations for Sargassum 

 floats gives an observed density of 42 floats/ 100 m^ 

 over the internal wave vs. 0.9 floats/100 m^ in front 

 of the set; the density over the internal wave is 

 significantly larger (46-fold greater) than the den- 

 sity in front of the set (Table 2). Again the data 

 indicate that Sargassum floats were carried shore- 

 ward by the internal waves. 



None of the types of larval fish characteristic of 

 water column samples (Table 3) were caught in any 

 neuston tow. Present in the neuston tows were only 

 those types of larval and juvenile fish which my 

 samples and the descriptions in the literature sug- 

 gest are characteristically neustonic. 



The densities of larval and juvenile fish frequent- 

 ly were significantly higher in the slick samples than 

 the samples from the rippled waters between slicks 

 (Table 4). On 24 June 1985, most larval fish, espe- 

 cially juvenile and postflexion stage larvae, were 

 rare in both the waters in front and behind the set 

 of internal waves. On this date in 6 tows, 3 in front 

 and 3 behind the set of internal waves, only 4 juven- 

 ile and postflexion larvae were caught as compared 

 to 185 in the 3 tows made in the slicks. Probably 

 because of the rarity of larval fish in both in front 

 and behind tows there were only three cases, total 

 and preflexion Hypsoblennius hentz and total lar- 

 val fish, in which the density of fish in front of the 

 set of internal waves was significantly higher than 



behind the set (Table 4). There are seven instances, 

 however, in which the density over the internal wave 

 of a larval or juvenile fish was significantly and at 

 least 10-fold higher than the density in front of the 

 set of internal waves (Table 4). In six of these in- 

 stances the fishes were at the juvenile, late post- 

 flexion, or early postflexion stages of development. 

 Larval and juvenile fish were grouped by stage 

 of development and the densities over and in front 

 of the internal waves were calculated for each 

 developmental stage (Fig. 2). The densities over the 

 internal waves of the juvenile, late postflexion, and 

 early postflexion developmental stages were signif- 

 icantly higher than the densities of these stages in 

 front of the set. There was not a significant differ- 

 ence between the densities over and in front of the 

 set of internal waves of flexion stage larvae and 

 preflexion larvae were significantly more abundant 

 in front of the set. These data suggest that internal 



1 1 1 — I I I I I I 1 1 1 — I I M I I 



0.1 1.0 10.0 



Density, No./IOOm^ 



Figure 2.— Densities of larval fish by stage of development caught 

 on 24 June 1985 in the waters in front of the set of internal waves 

 (open circles) and over the internal waves (closed circles). Data are 

 presented as the mean + 95% confidence interval with the points 

 above or below this line being the actual observations. Asterisks 

 indicate cases where the in front density was significantly different 

 (Wilcoxon's two-sample test, P < 0.05) from the density over the 

 internal waves. The method of calculating the densities is described 

 in the text. 



710 



