Squire Aerial monitoring of abundance of pelagic fishes 



359 



1972). MacCall & Prager ( 1988) presented graphics on 

 a time-series of abundance indices for jack mackerel 

 larva off southern California. They noted that the in- 

 dex did not correspond well with other larval indices 

 that included samples from central California. An ex- 

 amination of the MacCall & Prager (1988) larva index 

 vs. the aerial index indicated a poor correlation (r=0.16, 

 not significant), particularly for the years 1977-78, a 

 period of marked increase in jack mackerel abundance 

 off southern California. 



Bluefin tuna No biomass estimates are available for 

 bluefin tuna in the northeast Pacific, which is part of a 

 Pacific-wide resource. Young year-classes (l-3yr) are 

 common in the northeast Pacific with some catches of 

 fish up to 6yr of age (Bayliff 1980). CPUE estimates 

 presented by Calkins (1982) of catch/boat-day (C/BD) 

 for the U.S. purse-seine fishery operating off Califor- 

 nia and Baja California, Mexico for the years 1963-80 

 in the area north of 29° N lat. were compared with the 

 aerial core-area mean index. Although both indices (C/ 

 BD and T/BAF) show a decline prior to 1980, the cor- 

 relation was not significant (r=0.033). Calkins (1982) 

 stated that "total catch may be the best indicator of 

 bluefin abundance." Total catch (north of 29° N lat.) as 

 given by Calkins (1982) was compared with the core- 

 area index, and the correlation was again not signifi- 

 cant (r=0.147). 



Summary and conclusions 



The aerial monitoring program has produced informa- 

 tion on the trend of apparent abundance for six spe- 

 cies of pelagic near-surface schooling species. The pro- 

 gram in operation since 1962 utilizes information 

 provided by commercial aerial fish spotters. Spotter 

 pilots operate under strong incentive to estimate ton- 

 nage accurately, since their compensation is based on 

 actual tonnage caught. The cost of the program is a 

 small fraction of the cost of other field assessment 

 methods currently used to develop apparent abundance 

 data, such as egg and larva surveys and acoustic sur- 

 veys. 



Data are presented in a relatively simple and un- 

 derstandable index format of tons observed/block area 

 searched (T/BAF). Annual indices (T/BAF) of apparent 

 abundance are calculated for the major species of im- 

 portance to the commercial purse-seine fleet operating 

 off southern California and northwestern Baja Califor- 

 nia, Mexico. 



All species have shown a fluctuating pattern of ap- 

 parent abundance over the 29 yr survey period ( 1962- 

 90). The northern anchovy is the dominant species 



observed, accounting for 89.7% of all tonnage observed; 

 chub mackerel is second with 6.1%; followed by jack 

 mackerel with 2.1%, and Pacific sardine, Pacific bo- 

 nito, and bluefin tuna with 2.1%. 



From inspection of abundance data, peak periods of 

 higher abundance for four of the six species (chub mack- 

 erel, jack mackerel, northern anchovy, and bluefin tuna) 

 occurred in 1977-79. Analysis indicated that species 

 more closely related in abundance over time are the 

 northern anchovy and bluefin tuna, both being at 

 higher levels of abundance during 1972-80. A regres- 

 sion was calculated for 1963-88, giving a significant 

 correlation (r=0.76, P<0.1). 



A number of comparisons were made between esti- 

 mates of biomass (spawning or total biomass) and the 

 aerial abundance indices for the northern anchovy. Pa- 

 cific sardine, and chub mackerel, and the apparent 

 abundance of bluefin tuna based on CPUE data. There 

 is a significant correlation with the aerial index, the 

 anchovy total biomass estimate, and the historical egg- 

 production index. Also, there was a slightly less but 

 significant correlation with the acoustic survey esti- 

 mates. The more significant correlations were found in 

 comparing the aerial index with the northern anchovy 

 larva index and the Pacific sardine larva index. For 

 the chub mackerel, a species recognized as having a 

 highly fluctuating record of abundance, the correla- 

 tions during the early years (1963-66) were very good 

 (aerial index vs. larva index and spawning biomass 

 index); in later years, correlations between the larva 

 index and the estimates of total biomass and the aerial 

 index were not significant. There is a lack of good data 

 relative to the abundance for other species of interest 

 (Pacific bonito, jack mackerel, bluefin tuna) although 

 some comparisons with the aerial data have been made 

 for Pacific bonito, giving a reasonable correlation us- 

 ing a 3yr lag period. Bluefin tuna C/BD (catch/boat- 

 day) and total catch for the northwest Mexico-southern 

 California fishery were compared with the aerial core 

 index and gave no significant correlation. 



Trends of aerial indices indicate that the pelagic 

 resources studied can be described in a state of chaotic 

 fluctuations (Jensen 1987) relative to apparent abun- 

 dance. The fluctuating record indicates that even short- 

 term predictions of abundance appear not to be feasible. 

 Management must rely on near real-time estimates 

 based on a measurement of a small fraction of the 

 resource, and what is important is the development 

 and use of more than one method or approach in evalu- 

 ating population status. None of the survey results 

 appear in complete agreement; however, utilizing sev- 

 eral approaches to provide "triangles of agreement" 

 (from a method used in navigation) can provide a con- 

 sensus for management action. 



