FISHERY BULLETIN: VOL. 81. NO. 1 



jured fluctuate around 4.8% of the number killed 

 directly, ranging from 3 to 7%. The problem of 

 stress-induced mortality or debility was explored 

 in a workshop of experts on large mammal physi- 

 ology and pathology, and research plans to ap- 

 proach this problem were developed. 6 Subse- 

 quently one aspect of this problem was examined 

 with dolphin specimens collected aboard tuna 

 vessels. 7 Reproductive tracts were examined for 

 evidence of spontaneous abortion, and muscle 

 tissue for myopathy; no evidence of either was 

 found. No estimates of the magnitude of such 

 effects have been made, and currently no re- 

 search is underway to investigate stress-induced 

 mortality. As a conservative measure, given our 

 limited knowledge, I assume in the estimates of 

 total dolphin mortality given here that all of the 

 injured dolphins subsequently die of their in- 

 juries. Thus estimates of total kill of dolphins are 

 the sum of the estimated numbers killed directly 

 and the numbers injured. 



Numbers of net sets made by the tuna purse 

 seine fleet have been recorded by the Inter- 

 American Tropical Tuna Commission (IATTC) 

 from logbooks kept by the fishermen (Table 3). In 

 the logbooks the type of each net set may be re- 

 corded, along with tuna catch, location, and 

 other information. The three major types of sets 

 are 1) those known to involve dolphins, 2) those 

 known not to involve dolphins, and 3) those for 

 which the data indicate neither the presence nor 

 absence of dolphins. Types of sets not involving 

 dolphins include "floating object sets" (e.g., a 

 rope, board, log, etc.), a "school fish set" (i.e., a net 

 set on tuna sighted at or near the surface), and a 

 "porpoise set." The logbook data are incomplete, 

 however, because some members of the fleet do 

 not report and because, in some cases, only lim- 

 ited information was recorded by the fishermen. 

 The logbook coverage rate, however, is high. 



The data in columns D, N, and U in Table 3 

 have only recently become available, and analy- 

 ses are proceeding to use this information di- 

 rectly to estimate the total number of sets made 

 on dolphins. Preliminary results for the total 

 numbers of sets for each year 8 are similar to 



6 Stuntz, W. E., and T. B. Shay. 1979. Report on capture 

 stress workshop, La Jolla. California, May 1979. Southwest 

 Fish. Cent. La Jolla Lab., Natl. Mar. Fish. Serv., NOAA, 

 Admin. Rep. LJ-79-28, 24 p. 



7 Cowan, D., and W. Walker. 1979. Disease factors in Ste- 

 nella attenuata and Stenella longirostris taken in the eastern 

 tropical Pacific yellowfin tuna purse seine fishery. South- 

 west Fish. Cent. La Jolla Lab., Natl. Mar. Fish. Serv., NOAA, 

 Admin. Rep. LJ-79-32, 21 p. 



8 Inter-American Tropical Tuna Commission. 1981. Tuna- 



Table 3.— Number of tuna purse seine sets. 1959-78, (D) known 

 to have been made on dolphins in the eastern tropical Pacific, 

 (N) known not to have been made on dolphins, and ( U) unknown 

 if made on dolphins (IATTC text footnote 8), with (is) estimates 

 of the total number of sets made on dolphins (Smith text foot- 

 note 3). Also shown are the numbers of observed fishing trips 

 and purse seine sets on porpoise from NMFS records. 



those given in column E of Table 3, but the re- 

 sults are not yet available in the stratified form 

 needed to estimate numbers killed, described be- 

 low. Earlier estimates of the total number of sets 

 made on dolphins (column Uoi Table 3) were ob- 

 tained indirectly for the years prior to 1970, 

 based on the catches of tuna, and include an 

 adjustment for nonreported sets. 



For the Period 1959-72 



Estimating the annual rate of dolphin kill dur- 

 ing the period 1959-72 is difficult because obser- 

 vations were few, especially in the early part of 

 the period; consequently, extrapolation of infor- 

 mation on kill rates is necessary. One effect on 

 rate of kill is the development and improvement 

 of the "backdown" dolphin-release procedure (Coe 

 and Sousa 1972; Barham et al. 1977), by which 

 the vessel moves in reverse during a short portion 

 of the purse seine retrieval, thereby pulling the 

 net out from under the dolphins. Barham et al. 

 (1977) reported that the "backdown" dolphin-re- 

 lease procedure was developed aboard one vessel 

 in 1959 and 1960, and transferred to a second ves- 

 sel in 1961. Subsequently, the use of the proce- 

 dure expanded rapidly within the fleet, although 



dolphin investigations. Background paper 6, prepared for 

 the 39th meeting of the IATTC, Paris. October 1981. Inter- 

 Am. Trop. Tuna Comm., c/o Scripps Inst. Oceanogr., La Jolla, 

 CA 92093, 17 p. 



6 



