Moltschaniwskyj and Doherty: Distribution and abundance of juvenile Photololigo 



303 



water by individual traps requires cautious interpre- 

 tation of abundance estimates (Choat et al., 1993). 

 There are four species of loliginid squid currently 

 recognized in the Townsville region: Sepioteuthis 

 lessoniana, Loliolus noctiluca, Photololigo sp. B, and 

 Photololigo sp. A. 1 There are currently no morpho- 

 logical descriptions of the two Photololigo species, 

 but they can be readily identified by using allozyme 

 electrophoretic techniques (Yeatman and Benzie, in 

 press). Previously both of these species have been 

 referred to as Photololigo (Loligo) chinensis (Jack- 

 son and Choat, 1992; Yeatman and Benzie, in press), 

 but neither correspond to P. chinensis from Thai- 

 land. 2 Electrophoretic analysis of a subset of juve- 

 niles collected during three months of the program 

 found that all Photololigo sp. A were found less than 

 33 km offshore and 90% of the Photololigo sp. B 

 were found 33 km or more offshore. 2 Because these 

 species are morphologically identical as juveniles, 

 we assumed that all individuals found at stations 

 less than 33 km offshore were Photololigo sp. A and 

 that Photololigo collected more than 33 km offshore 

 were Photololigo sp. B. Photololigo sp. A (previously 

 known as Loligo chinensis) has been the topic of 

 recent growth studies using statolith aging tech- 

 niques (Jackson and Choat, 1992). This species is a 

 small short-lived neritic squid. Individuals are ap- 

 proximately 60 days old when they appear in the 

 adult population and they can grow to 180 mm in 

 120 days. Little is known about the early life-his- 

 tory and juvenile distribution patterns of either 

 Photololigo species. The objectives of this study were 

 to describe the spatial and temporal distribution 

 patterns of juvenile Photololigo species across the 

 continental shelf in the Townsville region of the 

 Great Barrier Reef. 



Materials and methods 



Sampling design 



Two major habitat types are found on the continen- 

 tal shelf, off Townsville, Australia. The inshore habi- 

 tat is a 56 km wide soft bottom coastal lagoon rang- 

 ing in depth from 15 m to 40 m. The offshore habi- 

 tat is a complex reef matrix of similar extent, dis- 

 sected by channels ranging from 40 m to 75 m deep 

 at the shelf break. To assess the cross-shelf distri- 

 bution of juvenile squid, four automated light-traps 

 (Doherty, 1987) were deployed at fifteen sampling 

 stations spanning the continental shelf and the 



1 C. C. Lu, Museum of Victoria, Australia, pers. commun. 1990. 



2 J. Yeatman, James Cook Univ., Australia, unpubl. data 1993. 



western Coral Sea (Fig. 1). Abundance along this 

 transect was assessed over four months, October to 

 January, during two austral summers, 1990/91 and 

 1991/92. At each station, the abundance of juvenile 

 squid was determined at two depths by deploying 

 two pairs of light-traps. In each pair, one light-trap 

 was suspended immediately below the surface while 

 the other light-trap was set deeper. In 1990/91, all 

 deep light-traps were suspended 20 m below the 

 surface. In 1991/92, the deep light-traps were sus- 

 pended within 5 m of the bottom to a maximum of 

 100 m in the Coral Sea. 



In all deployments, the two pairs of light-traps 

 were released approximately 300 m apart and al- 

 lowed to drift for one hour. Allowing the traps to 

 drift in the water minimized potential problems with 

 differential water movement among stations. The 

 use of drifting light-traps has been shown to be a 

 more effective way of catching pelagic organisms 

 than anchored light-traps in open water (Thorrold, 

 1992). After one hour, the four light-traps were re- 

 trieved and the entire catch was fixed and preserved 

 in 100% ethanol. Each evening the first light-trap 

 was deployed after 1930 hours (Eastern Standard 

 Time) and the last light-trap retrieved before 

 0430 hours. Travel time between each station al- 

 lowed only five cross-shelf stations to be sampled per 

 night. Thus, each night's activity concentrated on 

 one of the two continental shelf habitats or the Coral 

 Sea. Each monthly cruise consisted of nine nights 

 during which time each of the 15 stations was 

 sampled three times. However, sea conditions were 

 not always favorable. Sampling effort at each sta- 

 tion is shown in Table 1. 



It was not logistically possible to sample all sta- 

 tions in each habitat simultaneously. Therefore, time 

 of night is confounded with station position. Hap- 

 hazard selection of the first station sampled each 

 night ensured that no station was consistently 

 sampled at the same time on all nights. Cruises 

 were scheduled to include the new moon because 

 this is the lunar phase when light attraction has 

 proved most effective for fishes and various inver- 

 tebrates (Milicich, 1992). Temperature and salinity 

 profiles of the water column were collected at each 

 station by using a Seabird Conductivity Tempera- 

 ture Device during the 1991/92 summer. 



Concurrent with the summer cross-shelf sam- 

 pling, light-traps were anchored within 100 m of the 

 southeasterly side (weather-side) of four reefs; 

 Keeper, Helix, Faraday, and Myrmidon, to sample 

 near-reef water (Fig. 1). The use of drifting light- 

 traps near the reefs was not possible. During the 

 summer of 1990/91, four light-traps were anchored 

 at each reef; three immediately below the surface 



