Seasonal hydrological changes in continental shelf waters off the west coast, North Island, New Zealand, and comments on fish distributions

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Abstract
Surface and subsurface temperature and salinity data were collected on eight trawling cruises along the west coast of the North Island of New Zealand, between January 1971 and March 1973. Salinity ranges were more variable inshore than offshore, and geographical salinity variations were greater than seasonal changes. Temperature ranges were greater inshore than offshore, and at the surface compared with the bottom. Surface and bottom temperatures showed definite seasonal trends with maxima in January‐March and a sharp drop between May and the minima in July. Bottom temperatures were more uniform in winter than in summer. Inshore bottom temperatures ranged from 17–20°c in late summer to autumn (March to May) to 12–13°c in winter (July to September). Summer maxima were similar along the entire coast, perhaps marginally higher in the north, while winter minima north of Cape Egmont were higher (14°c) than south of the Cape (12.5–13.5°c). Offshore, bottom temperatures at Ninety Mile Beach and Cape Farewell were similar to each other during summer and autumn at 14–16°c, but warmer at Ninety Mile Beach by l‐2°c during the rest of the year. Bottom temperatures at 100 m in North‐west Trough (Cook Strait) were similar to those at adjacent inshore localities in winter and spring, but with a lower summer‐autumn maximum. Upwelling is regularly indicated along the coast, originating mainly in the top 100 m, but occasionally deeper. It may persist for 6–8 weeks in some areas. Development of the summer thermocline is described; it occurred at 10–50 m in summer, 30–75 m in autumn. Variations in mixed layer depth, change of temperature, and rate of change of temperature through the thermocline are sufficiently large to preclude predictive equations. The hydrological uniformity of this region, both in space and time, is reflected in the wide distribution of most demersal fish species. Annual variation in summer surface warming is associated with changes in distribution and abundance of juvenile albacore and southern bluefin tuna.

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