Climatic variability and the anchovy/sardine ratio in Hellenic waters

Abstract

In the present work, the monthly anchovy (Engraulis encrasicolus) and sardine (Sardina pilchardus) commercial catches, the anchovy/sardine commercial catch ratio as well as different climatic parameters in Hellenic waters (air and sea surface temperature AIRT and SST, sea level pressure SLP, N-S wind component NSW, and wind speed cubed WISC) over the 1964 to 1989 period were analyzed using spectral and cross-spectral analyses. The monthly anchovy and sardine catches as well as the anchovy/sardine catch ratio all exhibited a pronounced seasonal cycle that is related to the behaviour of sardine and anchovy and the nature of the purse seine fishery. Spectral analysis of the monthly sardine catches and anchovy/sardine catch ratio in the Hellenic waters also revealed cycles of 2.7 yr whereas that of anchovy catches revealed a prominent peak at 4.6 yr and secondary ones at 3.6 and 1.9 yr. Cross-spectral analysis resolved significant coherence between the anchovy/sardine ratio and SLP at 2.7 yr and marginally with NSW and WISC at 2.7 and 5.3 yr, respectively. The corresponding phase spectra showed that increased SLP and NSW over the north Aegean Sea proceed and are associated with decreased anchovy/sardine ratio with a lag of about 2.5 months for SLP and 6 months for NSW whereas increased WISC over the north Aegean proceed and are associated with increased anchovy/sardine ratio with a lag of 28 months. Cross-spectral analysis also resolved significant coherence between the anchovy catches and AIRT and SLP at 2.7 yr, NSW at 1.8 yr and WISC at 5.3 yr. The corresponding phase spectra showed that increased SLP, AIRT and NSW over the north Aegean Sea proceed and are associated with decreased anchovy catches with a lag of about 2 months for SLP, 16 months for AIRT and 6 months for NSW whereas increased WISC over the same area proceed and are associated with increased anchovy catches with a lag of 30 months. In contrast, cross-spectral analysis resolved no significant coherence between sardine catches and all five climatic variables. Possible mechanisms underlying such patterns are discussed.