Length-weight relationships of 55 mesopelagic fishes from the eastern tropical North Atlantic: Across- and within-species variation (body shape, growth stanza, condition factor).

We present estimates of length-weight relationships (LWRs) of 55 mesopelagic fish species of 13 taxonomic families based on data collected in the eastern tropical North Atlantic (ETNA) in March/April 2015. Our data include novel records for 19 species, while for 25 species LWRs are based on the most robust sample sizes, and for 21 species they are based on the most representative size ranges available up to now. In 31 species, body lengths were within the maximum range of body lengths recorded in the area, with new records of maximum lengths for 13 species. Most values for b fell between 2.5 and 3.5 with a mean exponent b of 3.08 (median 3.12) and a mean a of 0.0172 (median 0.0113). Body shape as covariate ('elongated', 'fusiform' and 'short-deep') strongly determined the variation in log a as a function of parameter b. For the mesopelagic fish species investigated, the form factor a indicated a significant increase of median a from 'elongated' to 'fusiform' to 'short-deep' body shapes. Large variability existed in parameter b between species of the same taxonomic family. Isometric growth was indicated in only nine species, whereas a positive allometry was suggested in 22 species. Using segmented regression analysis, we investigated ontogenetic variation in LWRs in 30 species. Of these, 20 species showed a breakpoint in LWR, whereby nearly equal numbers exhibited an increase or a decrease in slope following the breakpoint. Seven out of nine species showed significant regional variation in the slope of the relationship of the relative condition factor K vs. body length between two or more regions of the ETNA [eastern and western part of the oxygen minimum zone (LO-E, LO-W), northern and central equatorial region (EQ-N, EQ-C)]. A conspicuous pattern was an increase in K with body size in the LO-E (in six out of eight species), whereas in the LO-W and the equatorial regions the majority of species showed a related decrease. These findings support the idea that growth patterns in mesopelagic fishes in tropical regions show species-specific ecological niche and life-history adaptations that are finely tuned to small-scale regional environmental conditions. Comparison of our data with those of other studies emphasises that, regarding the small adult sizes of many mesopelagic fish species, estimates of LWR parameters are strongly influenced by sampled size distributions.