TANE 24, 1978 MARINE NOTES ON HEN ISLAND, NORTH-EASTERN NEW ZEALAND by Roger V. Grace and Anne B. Grace 274a Birkdale Road, Birkenhead, Auckland 10 SUMMARY Sea surface temperatures close to Hen Island dropped from 14.2 C to 13.8 C between 24 August and 1 September, 1977, and were approaching the expected winter minimum. Temperature patterns support the idea that the warm East Auckland Current passes to the east of Hen Island. Sublittoral fauna and flora is generally typical of that occurring along the Northland east coast. Crayfish were surprisingly rare, and fish populations lacked the diversity and abundance that might be expected in the area. The observations of Trevarthen (1954) on intertidal ecology of Hen Island are confirmed. INTRODUCTION Hen Island is the largest island of the Hen and Chickens group, situated in the outer Hauraki Gulf of north-eastern New Zealand. During a visit to the Island in August, 1977 (Hayward 1978), limited observations were made of marine life and sea temperatures. SEA-SURFACE TEMPERATURES Measurements of sea-surface temperatures were made between Whangarei Heads and Hen Island. Measurements were taken using a mercury thermometer and bucket dipped over the stern of the launch carrying members of the Hen Island Party to the Island on 24 August, and on the return trip on 1 September, 8 days later. Temperatures were recorded at five-minute intervals, allowing the position of records to be calculated assuming constant speed of the launch. Results are presented in Tables 1 and 2. Although temperatures were slightly higher towards Hen Island than close to the mainland coast, there was no abrupt discontinuity as experienced in the Bay of Plenty between Tauranga and White Island (Grace 1975), between Mangonui and Moturoa Island (Grace 1977), and between Tutukaka and the Poor Knights Islands (pers. obs.). In the latter areas, a discontinuity is often encountered apparently between the cooler coastal waters, and the warmer waters of subtropical origin carried in the East Auckland Current. It is likely that the main body of the East Auckland Current is deflected to the east near Bream Head (Paul 1968), following approximately the bathymetric contours, and passing to 131
Table 1. Sea surface temperatures, Whangarei Heads to Hen Island, 24 August 1977. Time Temperature ( C) Comments 1200 13.8 50m S of Frenchman Island. 1205 13.8 1210 13.8 1215 13.9 1220 13.9 1225 13.7 Colour change green to blue. 1230 13.7 1235 13.7 1240 13.8 1245 13.9 1250 14.0 1255 14.2 1300 14.2 Temperature constant. 1345 14.2 1km SW of NW tip of Hen Island. Table 2. Sea surface temperatures, Hen Island to Whangarei Heads, 1 September 1977. Time Temperature ( C) Comments 1300 13.9 1 km W of Dragon Mouth Cove. 1305 13.8 Water blue. 1310 13.8 1315 13.8 1320 13.8 1325 13.9 1330 13.8 1335 13.8 1340 13.8 1345 13.7" 1 1350 13.7 1355 13.6 1400 13.5 1405 13.5 I 1410 13.4 Water slowly becoming greener. 1415 13.5 1420 13.5 1425 13.4 1430 13.4, 1435 13.6 600m E of Busby Head. 1440 13.4 100m S of Frenchman Island. 1445 13.3 600m S of Home Point. 1450 13.4 200m SW of Urquharts Bay. 132
the east of the Hen and Chickens, Great Barrier and the Mercury Islands. Over the 8 day period between 24 August and 1 September, a drop in temperature of about 0.3 C occurred in the waters near Hen Island, with the lowest temperature near the Island of 13.8 C indicating that temperatures were approaching the winter minimum of about 13.0 to 13.5 C (Paul 1968). UNDERWATER OBSERVATIONS Diving was limited to one snorkel dive and one scuba dive because of cold and windy weather. Shallow water areas at Dragon Mouth Cove on the western end of Hen Island were briefly examined by snorkelling, and a scuba dive to 30 metres depth was made off the northwestern tip of Hen Island. Sublittoral flora and fauna is generally typical of that occurring along the Northland east coast, with a dense bed of the common kelp Ecklonia radiata dominating rocky bottom areas from shallow water to 30 metres. Some shallow areas are devoid of large algae, where considerable numbers of the algae-grazing echinoid Evechinus chloroticus occur. The lack of crayfish (Jasus edwardsi) was surprising considering that there were abundant suitable habitats available. Only a single female specimen was found despite considerable searching. No commercial pots were seen in the area. FISHES OBSERVED The following list of fishes observed includes 34 species, of which 32 are useful for comparative purposes (Nicholson et al. in press). According to standardised lists presented by Nicholson et al. of data from several sources, the 32 species at Hen Island compares with 31 species from Whangateau (Grace 1971), and 38 species from Tairua (Grace 1974). The greatest recorded diversity of fishes in New Zealand occurs at the Poor Knights Islands where 64 species (as standardised by Nicholson et al.) have been recorded (Russell 1971, Russell and Ayling 1976). Although with increased number of dives in the area, and increased effort to cover a wider range of habitats, several additional species could be expected to be observed at Hen Island, experience gained elsewhere suggests that the general abundance and variety of fish life was below the standard that could be expected in this area. It is suggested that this lack of diversity may be at least partly explained by the relatively weak influence of the East Auckland Current at Hen Island, when compared to, say, the Poor Knights Islands. Diversity of habitats is also reduced compared to the Poor Knights. The degree to which reef fish populations have been exploited by spearfishing or gill-netting at Hen Island is an unknown factor which may have influenced fish populations. Line fishing was also extremely poor in the area. 133
Family Myliobatidae Myliobatus tenuicaudatus Eagleray Family Muraenidae Gymnothorax prasinus Yellow moray Family Trachichthyidae Hopolostethus elongatus Slender roughy Family Zeidae Zeus australis John dory Family Acanthoclinidae Acanthoclinus quadridactylus* Rockfish* Family Serranidae Caesioperca lepidoptera Butterfly perch Family Carangidae Caranx georgianus Trevally Decapterus koheru Koheru Family Mullidae Upeneichthys porosus Red mullet Family Sparidae Chrysophrys auratus Snapper Family Kyphosidae Kyphosus sydneyanus Silver drummer Scorpis aequipinnis Blue maomao Girella tricuspidata Parore Family Pempheridae Pempheris adspersa Bigeye Family Aplodactylidae Aplodactylus meandratus Marblefish Aplodactylus sp. Notch-headed marblefish Family Cheilodactylidae Cheilodactylus spectabilis Red moki Nemodactylus douglasi Porae Family Chironemidae Chironemus marmoratus Kelpfish Family Pomacentridae Chromis dispilus Demoiselle Parma microlepis Black angelfish Family Odacidae Odax pullus Butterfish Family Labridae Pseudolabrus celidotus Spotty P. fucicola Banded parrotfish P. luculentus Orange parrotfish P. inscriptis Green parrotfish Coris sandageri Sandager's parrotfish Verreo oxycephalus Red pigfish Family Tripterygiidae Tripterygion varium Mottled blenny T. bucknilli Banded blenny T. sp. Yellow/black blenny T. sp. Oblique swimming blenny Family Gobiesocidae Trachelochismus sp* Clingfish* Family Balistidae Parika scaber Leatherjacket *denotes species not considered for comparative purposes. INTERTIDAL OBSERVATIONS The intertidal ecology of Hen Island, relating in particular to the southern shore, was described by Trevarthen (1954). His description was found to be consistent with our own observations in the area, and we have little to add to his commendable treatment of the subject. It is noted, however, that the crab Cyclograpsus insularum was probably lumped together with C. lavauxi by Trevarthen, a point noted by Saies (1973) for Trevarthen's description of boulder beach life on Little Barrier Island (Trevarthen 1954). The two species were not distinguished in most ecological studies until more recent work (Bacon 1971) helped to establish the distribution and relationships of these crabs. On the boulder beaches of Hen Island, the distinctive tan-coloured C. insularum is much more common than the finely spotted C. lavauxi, the latter species being 134
more characteristic of mainland boulder shores protected from heavy wave exposure. REFERENCES Bacon, M.R. 1971: Distribution and ecology of the crabs Cyclograpsus lavauxi and C. insularum in northern New Zealand. New Zealand Journal of Marine and Freshwater Research 5(3&4): 415-426. Grace, A.B. 1974: A preliminary checklist of fishes from the Tairua Region, northeastern New Zealand. Tane 20: 21-24. Grace, R.V. 1971: A checklist of fishes from the entrance to the Whangateau Harbour, Northland, New Zealand. Tane 17: 129-136. Grace, R.V. 1975: White Island notes. Tane 21: 91-100. Grace, R.V. 1977: Sea surface temperatures between Mangonui and the Moturoa Islands, northeastern New Zealand, in May 1976. Tane 23: 67-69. Hayward, B.W. 1978: Hen Island (Taranga) scientific camp, August 1977. Introduction and Acknowledgements. Tane 24: (this issue). Nicholson, J.; Dollimore, J.M. & Willan, R.C. (in prep.): A diving survey of fish populations in Matai Bay, Northland, New Zealand. Paul, L.J. 1968: Some seasonal water temperature patterns in the Hauraki Gulf, New Zealand. New Zealand Journal of Marine and Freshwater Research 2(3): 535-558. Russell, B.C. 1971: A preliminary annotated checklist of the fishes of the Poor Knights islands. Tane 17: 81-90. Russell, B.C. & Ayling, A.M. 1976: New records of fishes from northeastern New Zealand. Journal of the Royal Society of New Zealand 6(3): 277-286. Saies, A.B. 1973: The intertidal life of Ruamahua-iti and Middle Island (Aldermen Islands). Tane 19: 1-11. Trevarthen, C.B. 1954: Features of the marine ecology of Little Barrier, Mayor, and Hen Islands. Tane 6: 34-60. 135