DISTRIBUTION OF MICRONEKTON 

 BY SEASONS 



The only series of cruises which could possibly 

 reveal seasonal changes in the micronekton of 

 standard night hauls is the series for the Pacific 

 coasts of Mexico and Guatemala — cruise TO-58-1 

 in part, and cruises TO-59-1, TO-59-2, TO-60-1, 

 and TO-61-1 — for whicli results are summarized 

 in figures 14 to 16. 



Except for TO-60-1, these cruises were plan- 

 ned to detennine certain seasonal changes in the 

 standing crop of micronekton, and other ocean 

 properties, in the Gulf of Tehuantepec (long. 95° 

 W.). Comparisons for this area are available for 

 the months January-February (TO-59-1 ; no data 

 for the center of the Gulf, because of bad weather) , 

 March- April (TO-61-1), May- June (TO-58-1), 

 and August-September (TO-59-2). The data in 

 figures 14 to 16 show that all three main compo- 

 nents of the micronekton were scarcer by volume 

 in this area in August-September than in March- 

 June (possibly January-June). This difference is 

 to be expected from tlie study by Blackburn (1962, 

 1963) of seasonal changes in the physical, chemical, 

 and biological oceanography of the Gulf. The 

 physical processes that cause shoaling of the pyc- 

 nocline in certain areas and intermittent vertical 

 mixing in parts of these areas are well developed in 

 winter, and nutrient-rich water brought to the sur- 

 face could be expected to result in a high crop of 

 micronekton by spring. On the other hand, the 

 eutrophic conditions decline through the spring, 

 and the micronekton crop of late summer sliould 

 be low. Tlie time required for a crop of micronek- 

 ton to grow has been discussed by Blackburn 

 (1966a). 



Off the west coast of Baja California, eutrophic 

 conditions (coastal upwelling) are generally most 

 pronounced in spring (Reid et al., 1958) . If a lag of 

 a few months is assumed between the appearance 

 of a pliysical eutropliic process and the appearance 

 of the re.sulting crop of micronekton (see previous 

 paragraph), an increase in micronekton volume 

 from winter through spring to late summer miglit 

 be expected. Tlie data for fisli and cephalopods 

 (figs. 14 and 16) are not inconsistent with this 

 hyi^othesis, Init tliey are generally scanty (except 

 for August-September). For crastaceans (mainly 

 Pleuroncodes planipes) the data in figure 15 sug- 

 gest a rather high abimdance throughout the 



whole period mentioned above. Longhurst (1967), 

 who analyzed occurrences of P. planipes in zoo- 

 plankton net hauls, drew a similar conclusion. 

 Practically no quantitative data are yet available 

 from micronekton hauls on occurrences of P. 

 planipes in October througli December, although 

 it was abundant in liauls made with the high-speed 

 net in December 1960. Data on P. planipes from 

 zooplankton net hauls also are scanty at this 

 period, and it is possible that the species is then 

 less abundant than during the rest of the year, 

 although the evidence is inconclusive (Longhurst, 

 1967). The distribution and ecology of this animal 

 are being further studied in the Scripps Tuna 

 Oceanograpliy Research Program. 



Elsewhere along the Mexican coast, namely 

 across the mouth of the Gulf of California and 

 thence southwards along the coast to the Gulf of 

 Tehuantepec, no hypothesis about seasonal distri- 

 bution of micronekton has been developed to test, 

 and the scanty data in figures 14 to 16 do not sug- 

 gest any particular seasonal change. 



COMPARISON OF MICRONEKTON 

 CAUGHT BY NETS AND TUNAS 



Alverson (1963a), who sorted stomach contents 

 of 2,846 yellowfin tuna '■ and 1,010 skipjack tuna '•• 

 from the eastern tropical Pacific, published tables 

 showing the percentage composition by volume of 

 this material by taxa (including some species) for 

 each of the areas 1 to 14 of figure 5. 



Alverson's specimens of tuna were obtained 

 from commercial catches made by surface hooking 

 (bait-and-pole) or surface netting (purse seine). 

 Yellowfin tuna are caught also by subsurface 

 hooking (pelagic long-line) in parts of the eastern 

 tropical Pacific, but few data on stomach contents 

 are available for fish from that region (Juhl, 

 1955; Blunt, 1960). 



Tables 8 to 14 compare the percentage composi- 

 tion of ac'tual volumes of micronekton from stand- 

 ard night hauls with the actual volumes of stom- 

 ach contents of yellowfin and skipjack tuna, for 

 areas in which at least five hauls were available 

 (namely areas 1, 2, 3, 4, 5, 6, and areas 11 and 12 



^ These two species of tuna were caUed, respectively. Neothun- 

 nus mncropterus and Katsuwonus pelamis, by Alverson ; accord- 

 ing to Collette and Gibbs (1963), they should be known as 

 ThUHHHS albacares (Bonnaterre) and Euthynnua pelamis 

 (Linnaeus). 



102 



U.S. FISH AND WILDLIFE SERVICE 



