micronekton net volume, is of the order 1 : 10. The 

 reasons are unknown; possibly red crabs are 

 scarcer in the upper 10 m. than at greater depths, 

 or they may tend to avoid a fast-moving ship. The 

 concentrations shown for this method in tables 1-6 

 have all been multiplied by 10 to make them 

 broadly comparable with those from the other 

 hauls. 



The remaining sources of information about red 

 crabs were sightings of aggregations at the sea 

 surface and occurrences in the stomachs of pred- 

 ators that were captured from the ship. The con- 

 centration for each sighting was obviously over 40 

 ml./l,000 m.^ and is listed accordingly (see S in 

 tables 1-6). On four occasions the stomachs of 

 predators contained crabs. The predators were yel- 

 lowfin tuna, dolphin, Coryphaena hippums, and 

 an unidentified turtle. Three of these occurrences 

 were represented by only one predator individual ; 

 it was not then safe to assume that red crabs oc- 

 curred at a concentration over 40 ml./l,000 m.^ In 

 the other occurrence, three predator individuals 

 (yellowfin tuna) containing red crabs were cap- 

 tured and many others seen ; here it was reasonable 

 to assume that a high concentration of red crabs 

 was available (see P in table 5) . 



For stations at which two or more estimates of 

 concentration were available, the highest one was 

 listed in tables 1-6. In chai-t.ing the isogram of 40 

 ml./l,000 m.^ concentration for each cruise, I used 

 the data of tables 1-6, together with the zero con- 

 centrations recorded for the remaining stations. 

 In view of the opinion of Boyd (1967) and Long- 

 hurst (1967) that red crabs make diurnal vertical 

 migrations, I occasionally ignored estimates of low 

 or zero concentrations based upon daytime catches 

 or observations when other evidence suggested the 

 probability of high concentrations in the area — for 

 instance when surface chlorophyll a was high, and 

 high concentrations of crabs were encountered dur- 

 ing one or both of the adjacent nights. An addi- 

 tional justification for this procedure is that most 

 of the daytime information was obtained from 

 zooplankton hauls, wliich frequently yield low es- 

 timates of concentration as shown above. On the 

 other hand, if a series of night observations all 

 yielded low estimates of concentration I concluded 

 definitely that red crabs were not abundant in tlie 

 area examined, whatever the other circumstances 

 were ; I sometimes ignored an occasional low con- 



154 



centration in a series of high ones obtained during 

 the same night. 



Stations 60-66 of cruise TO-64-1 were occupied 

 very close to each other, in an area of about 5 

 nautical miles (9 km.) radius, during a period 

 of about 3 days. The odd-numbered stations were 

 occupied about noon, the even-numbered stations 

 about midnight. Table 1 shows the crab concen- 

 trations measured in zooplankton hauls at these 

 stations which were all much higher than 40 

 ml./l,000 m.^ Concentrations were lower at noon 

 than at midnight, as expected. Concentrations in 

 consecutive hauls diflFered by a factor of about 

 three in both tlie noon and midnight series. I have 

 found similar differences for other kinds of orga- 

 nisms in noon and midnight series of micronekton 

 hauls tliat were closely adjacent in space and time 

 (Blackburn, 1968). Concentrations of red crabs 

 estimated from single net hauls might, therefore, 

 differ by a factor up to three from concentrations 

 actually present in the water. I tried to allow for 

 this possibility in charting the isogram of 40 

 ml./l,000 m.^ for each cruise. 



TUNAS 



Data on the recorded occurrence of yellowfin 

 and skipjack tunas were obtained from lATTC, 

 which has such data in various forms, one of which 

 gives average catch per unit of fishing effort by 

 species, by fishing method (live-bait and purse- 

 seine), location by 1° .squares, and by months. 

 Such information had been very useful in pre\'ious 

 work on tuna ecology in the Gulf of Tehuan tepee 

 (Blackburn, 1963) but was rather unsatisfactory 

 for this study. The isograms of temperature, 

 chlorophyll a, and red crabs intersected the paral- 

 lels and meridians in a way that was frequently 

 very complicated (e.g., fig. 6), and it was evident 

 that their position did not always remain un- 

 changed for the whole of the calendar month in 

 which they were observed. 



I used lATTC data on tuna occurrences that 

 liad definite dates and positions. Figures 4, 6, 8, 

 10, 12, and 14 (see later sections) show these oc- 

 currences separately for each species and for mixed 

 catches of the two species. They refer eitlier to the 

 whole period when the ship was in the area of the 

 charts or to some part of that period, as explained 

 later for cacli cruise. The original data were avail- 

 able from baitboats and jjurse seiners separately, 

 but the distributions of tuna from these two 



U.S. FISH AND WILDLIFE SERVICE 



