NOTE GardunoArgueta and Calderon-Perez: Depth distribution of Penaeus brev/rosms 



401 



respectively), except for a small catch in the former 

 at the 9-15 m depth interval (0.13 ±0.19 ind./ha). In 

 mid-June, very few specimens were collected (0.37 

 ±0.26 ind./ha), all in the Presidio and Baluarte 

 transects at depths of about 10 m (Fig. 3C). During 

 the summer cruise (August 1986) all individuals were 

 caught at the deeper stations, (>70 m, 1.50 ±0.73 ind./ 

 ha; Fig. 3D). 



The distribution of P. brevirostris observed during 

 the BIOCAICT cruises, at all the stations, appeared 

 to be limited to a narrow temperature range. The 

 majority of individuals were caught at temperatures 

 between 14.1 and 18.0°C and, although some indi- 

 viduals were also found at about 24.0°C, the average 

 mean abundance at this temperature was rather low 

 (0.1 ind./ha; Fig. 4). 



Similarly, the main distribution of P. brevirostris 

 appears to be limited to a narrow bottom dissolved 

 oxygen range (Fig. 5). Mean shrimp densities were 

 highest at the DO levels of 0.9-1.5 mL 0,/L while at 

 higher (2.9-4.1 and 4.2-6.0 mL O/L) and lower lev- 

 els (0.1-0.8 mL O^), mean shrimp abundance was 

 lower ( Kruskal-Wallis test, P<0.005 ). Crystal shrimp 

 abundance was significantly higher at the DO inter- 

 val 0.9-1.5 mL/0 2 than at the other DO intervals 

 (NMRT, P<0.005. Abundance differences among 

 other DO intervals were not significant. 



The former suggests that P. brevirostris occupies 

 waters with lower oxygen content than do the other 

 members of the genus. Most studies emphasize their 

 distinct avoidance of hypoxic water and the nega- 

 tive consequences of exposure to it. Renaud (1986) 

 found that P. aztecus and P. setiferus were able to 

 detect and avoid hypoxic water below 2 ppm (1.4 mL 



2 /L) and 1.5 ppm (1.05 mL 2 /L), respectively. 

 Broom (1971) found a slightly higher threshold for 

 the latter species (2 ppm, 1.4 mL O^). Rubright et 

 al. ( 1981) and Bassanesi-Poli ( 1987) reported surface 

 oriented movements of P vannamei when dissolved 

 oxygen in culture ponds dropped below 2 mlVL. Egusa 

 andYamamoto( 1961) observed stress in P japonicus 

 for oxygen concentrations below 2 mL/L and evacua- 

 tion of their burrow at a DO level of 0.7 ppm (0.5 mL 

 Og/L). Regarding prolonged effects of hypoxia, Clark 

 (1986) reported that P. latisulcatus showed molt in- 

 hibition and increased mortality under the hypoxic 

 conditions of 2 ppm (1.4 mL O^). 



The disappearance of P. brevirostris from tradi- 

 tional fishing grounds in the summer may be due to 

 the occurrence of temperature and dissolved oxygen 

 levels outside its tolerable range. The population may 

 move to deeper regions where low temperature and 

 DO conditions, to which they are better adapted, 

 prevail. Calderon-Perez 3 found a substantial num- 

 ber of shrimps of this species 120 m deep in June 

 1992 but none in shallower areas where tempera- 

 ture and DO were higher. 



Thus, the seasonal variation in depth distribution 

 of P. brevirostris in the eastern Pacific ocean may be 

 due to bathymetric movements which, for most ani- 

 mals (Figs. 4 and 5), seem to be determined by sea- 

 sonal changes in environmental parameters. 



3 Calderon-Perez, J. A. 1992. Data from the BIOCAPESS VI cruise 

 on board the B/O (R/O) El Puma, carried out between 24-30 

 June 1992. Estacion Mazatlan, Institute de Ciencias del Mar y 

 Limnologia, U.N.A.M., apdo. Postal 811, Mazatlan, Sinaloa, 

 Mexico, CP 82000. 



11.0 15.0 19.0 23.0 27.0 31.0 



Temperature (°C ) 

 Figure 4 



Mean abundance (shrimp/ha ±SE) of crystal shrimp, 

 Penaeus brevirostris, in relation to bottom temperature 

 (°C). Vertical lines represent standard error of the mean 

 for each 2°C interval. 



10 2.0 3.0 4.0 5.0 60 



Dissolved Oxygen (mL/L) 

 Figure 5 



Mean abudance (shrimp/ha ±SE) of crystal shrimp, 

 Penaeus brevirostris, in relation to dissolved oxygen (DO) 

 concentration. Vertical lines represent standard error of 

 the mean calculated for each DO interval of 0.5 mL/L. 



