Long-term changes in the abundance and deepening of the deep-sea shrimp Aristaeomorpha foliacea in the Balearic Basin: Relationships with hydrographic changes at the Levantine Intermediate Water

We compiled long-term data (from commercial landings and scientific surveys) on the density (D) and the depth distribution of the deep-living shrimp Aristaeomorpha foliacea in the Balearic Basin (western Mediterranean) from the 1950s to 2010. We analyzed these data in relation to changes in temperature (T) and salinity (S) of the intermediate water column, where the species inhabits. A. foliacea showed the highest densities in the 1950s–1960s in the northern sector of the Balearic Basin off the central coasts of Catalonia, ca. 0.6–20.7ind/ha, and to the north of Mallorca, ca. 7.5–43ind/ha. Later, densities>1ind/ha were only found close to Eivissa Channel at the south side of the Balearic Basin: 1.9–11.4ind/ha in 1992, 2001 and 2005. In the northern Balearic Basin, A. foliacea has almost disappeared since the 1970s. In the same period A. foliacea showed significant deepening (r²=0.90) of its population Center of Gravity (CoG) from the 1950s–60s (CoG=466m) to 1995–2002 (CoG=538m) and then stabilized during 2003–2010 (CoG=535m). Temperature of the Levantine Intermediate Water (LIW) core (at ca. 400m depth) off the central coasts of Catalonia progressively increased from 12.95°C in 1957 to 13.31°C in 1977, reaching 13.44°C and 13.41°C in 1998 and 2007 respectively. Salinity showed a similar increase from 38.40psu in 1957 to 38.43psu in 1977, then reaching 38.55psu in 1998 and 38.56psu in 2007. Estimated D of A. foliacea decreased significantly (Spearman ρ) when the T and S at the LIW core increased (p<10⁻⁶), as did T and S at the optimum depth of A. foliacea (p<0.001). Multiple Linear regression (MLR) models related the decrease of A. foliacea D with increasing T at LIW (up to 55% of explained variance, depending upon the variables included in the MLR model). The optimum depth of A. foliacea was always deeper than the LIW core, with the depth difference between the CoG and the LIW core increasing from 80.6m in 1957 to 138.1–153.7m in the 1990s–2000s. Thus, the optimum habitat of the species was never directly linked to LIW conditions (maxima of T and S) as previously thought. We discuss possible physiological and trophic effects that could explain the local extinction of A. foliacea in the northern Balearic Basin due to changes of T and S in its habitat, associated with reduced O₂ concentration at the LIW core.