(This exercise is based on Lehodey, P., M. Bertignac, J. Hampton, A. Lewis, and J. Picaut. 1997. El Niño Southern Oscillation and tuna in the western Pacific. Nature 389: 715–718.)
(Note: The reference above links directly to the article on the journal’s website. In order to access the full text of the article, you may need to be on your institution’s network [or logged in remotely], so that you can use your institution’s access privileges.)
As we learned in the textbook, the El Niño Southern Oscillation (ENSO) is a cyclical pattern of changes in the temperature of the Pacific Ocean near the equator. Affecting climate across much of Earth, ENSO usually cycles with a periodicity of 2–7 years, although the magnitude and period of its cycles can vary considerably. During the El Niño phase of ENSO, the waters of the eastern equatorial Pacific are unusually warm; in the La Niña phase, these waters are unusually cold.
On the eastern edge of the warm water pool in the western Pacific is a zone where warm, low-salinity water meets cooler, high-salinity water. This convergence zone moves to the east during the El Niño phase of ENSO and to the west during the La Niña phase of ENSO.
Because plankton and the somewhat larger micronekton often aggregate in the convergence zone, it is reasonable to assume that large predators, such as tuna, would also congregate in this zone. Do tuna track movements of the convergence zone associated with ENSO? Can we use information from water temperature changes associated with ENSO to predict where tuna fishers should direct their effort in order to maximize their catches?
Lehodey and colleagues from the Oceanic Fisheries Programme of New Caledonia explored these questions. They used the 29°C sea surface temperature (SST) isotherm as a proxy for the convergence zone, as previous studies have shown that movement of this isotherm is highly correlated with the convergence zone. Relative tuna abundances were estimated from catch per unit effort (CPUE) data collected from commercial fisheries.
Figure 1 The distributions of tuna catches (black circles: larger circles denote larger catches) and mean SST isotherms. The upper panel (a) shows the distributions during the first half of 1989, when the ENSO was in a La Niña phase. The lower panel (b) shows the distributions for the first half of 1992, when ENSO was in an El Niño phase.
Question 1
How far east did the 29°C SST isotherm reach in 1989? In 1992?
Question 2
At the equator, one degree of longitude is approximately 110 km. Over what distance (in km) was the isotherm displaced between 1989 and 1992?
Question 3
Describe the changes in the locations in which tuna were caught in 1992 as opposed to 1989.
Figure 2 The pink line represents the center of gravity of CPUE, and the blue line represents the 29°C isotherm. These lines are on the same longitudinal scale (see bottom of figure). The broken red line is the SOI, drawn to a different scale (see top of figure).
Question 4
The researchers tracked the center of gravity for the catch per unit time (CPUE) of tuna and the 29°C isotherm between 1988 and 1995. The center of gravity is essentially the average location of tuna catches. They also tracked the Southern Oscillation Index (SOI), a difference in air pressure between Tahiti and Darwin (Australia), which characterizes phases of ENSO. The El Niño phase is associated with strong and sustained negative values of SOI, while the La Niña phase is associated with strong and sustained positive SOI values. Describe the patterns you observe in the changes in the isotherm and the SOI over time.
Question 5
Describe the patterns you observe in the center of gravity of the CPUE in relation to the 29°C isotherm.
Figure 3 The correlations between the isotherm and the SOI with various lag times (in months). Correlations that are above or below the two dotted lines are statistically significantly different from 0. (A correlation value is an indication of the strength of the relationship between two variables. For more on correlations and other statistical values and techniques, see the Web Stats Review.)
Question 6
The researchers wanted to more rigorously examine the correspondences between the isotherm and the SOI, and the center of gravity of the CPUE and SOI. Additionally, they wanted to see whether changes in one of these factors preceded changes in the other; and if so, by how much. Thus, they calculated correlations between the SOI against values of the isotherm that were displaced by different amounts of lag time. A lag time of zero means that the values for the SOI and the isotherm were not displaced in time. A positive lag time means that isotherm values were tested against SOI values that occur later in time (e.g., July 1992 isotherm versus October 1992 SOI is a positive 3-month lag time). A negative lag time means that the isotherm was tested against SOI values that occur earlier in time (e.g., July 1992 isotherm versus April 1992 SOI is a negative 3-month lag time). For what value of lag is the correlation between the isotherm and SOI most negative? What inferences can you draw about using changes in the isotherm to predict changes in SOI?
Figure 4 The correlations between the isotherm and the center of gravity for CPUE with various lag times (in months). Correlations that are above or below the two dotted lines are statistically significantly different from zero.
Question 7
For what value of lag is the correlation between the isotherm and the CPUE center of gravity most negative? What inferences can you draw about using changes in the isotherm to predict changes in tuna catch?
Figure 5 (a) Tuna released April 1991, recaptured before February 1992. (b) Tuna released May 1991, recaptured before February 1992. (c) Tuna released March 1992, recaptured before October 1992. For clarity, only tagged fish recaptured in the zone 10°N–10°S and more than 10° of longitude east or west of their release location are plotted. Tagging data were compiled from records of a large-scale tagging program carried out by the South Pacific Commission during 1990 to 1992 (see ref. 30 in the original paper). Thick arrows indicate the direction and magnitude of displacement of the skipjack CPUE gravity centre (see the Methods section of the original paper) during the tag recapture periods.
Question 8
To further test whether the tuna were actually moving with the isotherm (and hence, the convergence zone), the researchers also looked at movements of tuna that had been tagged at three different periods of time. What patterns of movement are apparent?
Question 9
Referring back to Figure 2, how do these patterns of movement correspond to ENSO cycles?
