|Empirical investigation on the relationship between climate and small pelagic global regimes and El Niño-southern oscillation (ENSO) (1997)|
|3. RESULTS AND DISCUSSION|
|3.2. Small pelagic regimes|
Figure 18 shows the RIS values, together with the catch records of each sardine and anchovy stock. From this figure, it seems evident that gross general parallel trends exist; sardines tend to be abundant worldwide when anchovies are at low population levels, and vice-versa. Despite these general trends, it is also evident that values and trends within the catch records of the various stocks do differ from one another, particularly when year-to-year changes are considered. Catch records are only a gross index of large abundance changes, moreover, variations are a function of global climatic factors linking the different areas, and also of particular effects that would tend to differ between regions. Therefore, neither the composite RIS nor any particular catch record should be expected to closely reflect the effects of interdecade climate variability of small pelagic populations.
Despite the limitations, we believe the RIS is a better indicator of decadal trends than any individual record, because any composite series would tend to emphasize the common long-term variability while reducing the effects of the individual year-to-year variations. Using standardized values when computing the RIS values tends to compensate for those stocks that reached very high abundances, such as the Humboldt anchovy and the Japanese sardine, as compared to those stocks of less productive systems as California.
Since more direct estimations of stock abundance have not been made for all the areas and for periods long enough as to reflect the interdecadal regime variations, the RIS (or any similar composite series based on catches) might be the only available data for assessing the effects of climate variability on pelagic resources on a worldwide basis. This type of evidence is considered useful for the study of interdecadal climate variability, given the lack of both a more complete knowledge of the physical mechanisms and of long records for some physical variables in poorly studied areas (Sharp and McLain 1992).
Figure 18 shows there is no clear tendency during the first years of the RIS series, a period when most fisheries were at their beginning. Therefore, catch of those years cannot be expected to reflect large abundance changes because of the low level of effort. After this period, sardines were abundant worldwide from 1925 to 1950, peaking around the mid-1930s. Thereafter, anchovies dominated from the early 1950s up to the late 1970s, peaking around the late 1960s. The late 1970s and most of the 1980s were again years of sardine abundance, but recently (early 1990s) the sardine dominance has been declining (remembering the Benguela stocks fluctuate out of phase with the others, see Lluch-Belda et al. (1989, 1992). Special considerations were used regarding these stocks when computing the RIS series. See data and methods section.)
We note, except for the first years of the RIS series, there has been no prolonged period when both sardines and anchovies show similar population levels, that is, the RIS values are either positive or negative but no near-zero values are observed for more than one consecutive year. Moreover, the alternation between sardines and anchovies stocks is an interdecadal variation, as previously noted by Kawasaki and Omori (1988) and Lluch-Belda et al. (1989, 1992).