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Correlation with Hydrographic Conditions
Although single surface water temperatures are in themselves a somewhat unreliable index of the underlying water mass, by averaging these temperatures in ten mile squares (each square overlapping adjacent squares by five miles) it has been possible to obtain mean monthly temperatures from which isotherms have been drawn to depict at least the gross features of monthly temperature variations (Fig. 11) (Broken lines represent isotherms where the data are less adequate) It is believed that these mean isotherms are approximately representative of the water mass temperature in the upper ten fathoms, since those taken in November, 1951, are in substantial agreement with a series of bathythermograph soundings taken in the last week of that month by N.Z. Oceanographic Institute.
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It will be seen that there is a marked difference in surface temperature in the Gulf between the two seasons. In November, 1950, there is a gradient from 18 5° C. near Rangitoto Channel to 17 5° C. across the mouth of the Gulf. In December the gradient remains the same, but the temperature has increased approximately one degree throughout. In November, 1951, the temperatures are at least a degree cooler than at the same time the prvious year, while in December the temperature gradient of the previous year is reversed in the inner portion of the Gulf, with a pocket of cold water lying along the western shore. It is not until January that the mean temperatures reach 18.0° C. in the western part of the Gulf. Table 7 summarises the general meteorological and hydrographic conditions for the two seasons Water temperatures are taken from the mean isotherms midway between Kawau and Tiritiri Islands, while the remaining data are from meteorological records for Auckland.
Fig. 11a.—Mean monthly isotherms, Hauraki Gulf, November, 1950.
Fig. 11b.—Mean monthly isotherms, Hauraki Gulf, December, 1950.
Fig. 11c.—Mean monthly isotherms, Hauraki Gulf, November, 1951.
Fig. 11d.—Mean monthly isotherms, Hauraki Gulf, December, 1951.
Fig. 11e.—Mean monthly isotherms, Hauraki Gulf, January, 1952.
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| Mean surface water temp. Degrees C. | Mean air temp Degrees C. | Rain days | Sunshine hours | Remarks | |
|---|---|---|---|---|---|
| November, 1950 | 18.0 | 17.4 | 10 | 168 | Mild easterly conditions |
| December, 1950 | 19 0 | 19.0 | 9 | 233 | Warm westerly conditions |
| January, 1951 | 19 7 | 11 | 179 | Mild settled weather | |
| November, 1951 | 16.7 | 16.2 | 15 | 205 | Unsettled westerly |
| December, 1951 | 17.0 | 16.7 | 16 | 204 | Cool and changeable |
| January, 1952 | 18.0 | 17.9 | 14 | 239 | Changeable westerly |
It is seen from these records that water and air temperatures are closely correlated and that in general the weather was warmer and more settled in the summer of 1950–51. Since the main bursts of spawning occurred in November, 1950, and January, 1952, it would appear that 18° C. may be a critical water temperature and that until the surface temperature rises to this level the majority of fish will not spawn. It will be noted, however, that the temperatures quoted are means about which a considerable variation may occur, even though the main water mass may be more constant. There is no apparent correlation between egg counts and temperatures for individual samples.
Unless the snapper spawns at the surface it will obviously be dependent on subsurface rather than surface temperature. However, the bathythermograph soundings mentioned above indicate that approximately the same temperature prevails from the surface to between 10 and 15 fathoms, an abrupt thermocline of up to 3° C. separating the warmer upper from the colder lower water mass. Since the “school snapper” in the early part of the spawning season are usually reported at depths of approximately 10 fathoms it would seem that the fish do indeed seek this upper layer about the time it reaches the necessary minimum temperature of 18° C. In future investigations it may be possible to amplify this hypothesis by correlating the depth of the thermocline with the depth at which snapper are taken. If, as seems likely, the fish rise only until they reach the lower boundary of the warm water and no more, such information might be of considerable use to the line fisherman in determining what length of line to set Cousteau (1953) describes similar conditions where the change in temperature is so sharp that an aqualung diver swimming in the warmer layer may extend his hand a few inches and readily feel the colder layer beneath. It would perhaps be possible for a fish to adjust its hydrostatic balance to float without effort on the thermocline.