For many mesopelagic fishes, even basic knowledge regarding their biology is missing, greatly impeding their effective management. Here we present length–weight relations for 16 mesopelagic fishes sampled during research surveys in the Greek seas (eastern Mediterranean). The following species were studied: Benthosema glaciale (Reinhardt, 1837); Ceratoscopelus maderensis (Lowe, 1839); Diaphus holti Tåning, 1918; Diaphus metopoclampus (Cocco, 1829); Diaphus rafinesquii (Cocco, 1838); Hygophum benoiti (Cocco, 1838); Hygophum hygomii (Lütken, 1892); Lampanyctus crocodilus (Risso, 1810); Lobianchia dofleini (Zugmayer, 1911); Myctophum punctatum Rafinesque, 1810; Notoscopelus elongatus (Costa, 1844); Symbolophorus veranyi (Moreau, 1888) [Myctophidae]; Argyropelecus hemigymnus Cocco, 1829; Maurolicus muelleri (Gmelin, 1789) [Sternoptychidae]; Stomias boa (Risso, 1810); Chauliodus sloani Bloch et Schneider, 1801 [Stomiidae].With the exception of Diaphus holti and Symbolophorus veranyi, parameter b diverged significantly from isometry. Only two species (Benthosema glaciale and Chauliodus sloani) displayed negative allometry, while for the remaining 12 species a positive allometry was found, with the highest parameter b values estimated for Stomias boa and Diaphus rafinesquii. The median value of parameter b for all species was 3.236 and 50% of its values ranged from 3.173 to 3.323. Some variations of the parameter b were observed between our findings and other studies from the Atlantic and the western Mediterranean.

LWR, Mediterranean Sea, mesopelagic fish, micronekton, myctophids, twilight zone

Mesopelagic fishes constitute the most abundant group of vertebrate animals on the planet (Irigoien et al. 2014) and represent a high diversity of species (López-Pérez et al. 2020) with the family Myctophidae having the higher number of species among them. These species inhabit the part of the ocean known as the mesopelagic zone, usually set between 200–1000 m of depth, also referred to as the twilight zone, forming Deep Scattering Layers detected in oceanic mid-waters by echosounders (Godø et al. 2009; Kaartvedt et al. 2019). The majority of the mesopelagic fish species are known for their diel vertical migrations to the epipelagic layer during the night, following the ascension of their zoo-planktonic prey to shallower oceanic depths, while during daytime they descend back to the mesopelagic zone to avoid predation (Kaartvedt et al. 2019).

Mesopelagic fishes play an important ecological role, linking primary consumers to top predators (Woodstock and Zhang 2022), many of which are commercial pelagic and demersal fishes (or protected, endangered, or threatened species) (Catul et al. 2011). Therefore, they constitute an important part of open ocean energy dynamics and contribute considerably to the transfer of organic carbon from the surface to the deep sea via their diel vertical migrations (Kaartvedt et al. 2019). Mesopelagic fishes have been regarded as a potential harvestable resource since the 1970s either for human consumption or as raw material supply to the fish meal and marine oil industry, but efforts in this direction were mainly exploratory or economically unsustainable (Caiger et al. 2021). In the recent decade, their global biomass estimate has been substantially revised upwards (Irigoien et al. 2014), and interest in commercial exploitation is being revisited. Despite their ecological importance and fisheries potential, these species remain one of the least investigated components of the marine environment. For many mesopelagic fishes, even basic knowledge regarding their biology is missing, greatly impeding their effective and sustainable management (Hidalgo and Browman 2019; Caiger et al. 2021).

Length–weight relations (LWRs) constitute essential knowledge for the application of fish stock assessment and management, necessary for the estimation of fish biomass from sampled length data and for ecological modeling and the estimation of growth in fish (Froese 2006). Length–weight relation studies for Mediterranean mesopelagic fishes have been particularly scarce (Battaglia et al. 2010), especially in the eastern basin, where information is almost absent. In the presently reported study, we estimated the length–weight relations for 16 mesopelagic fish species, representing 3 families, sampled during research surveys in the Greek seas.

Fish samples were collected with pelagic trawls and midwater frames, during dedicated mesopelagic surveys as well as other routine acoustic surveys (Leonori et al. 2021), onboard the R/V Philia in the Greek seas (North Aegean Trough, northern Euboean Gulf, Saronic Gulf, Cretan Sea, Gulf of Corinth) (Fig. 1) from November 2018 to December 2019. Sampled fish were immediately packed and frozen onboard until their transfer to the laboratory for examination, where the total length (TL) of each individual was measured to the nearest 1 mm and the total weight (W) to the nearest 0.001 g, using a high precision digital scale. The LWRs were estimated for 16 fish species, 12 of which represented the family Myctophidae, two species to the family Sternoptychidae, and two to the family Stomiidae (Table 1). The following species were studied: Benthosema glaciale (Reinhardt, 1837); Ceratoscopelus maderensis (Lowe, 1839); Diaphus holti Tåning, 1918; Diaphus metopoclampus (Cocco, 1829); Diaphus rafinesquii (Cocco, 1838); Hygophum benoiti (Cocco, 1838); Hygophum hygomii (Lütken, 1892); Lampanyctus crocodilus (Risso, 1810); Lobianchia dofleini (Zugmayer, 1911); Myctophum punctatum Rafinesque, 1810; Notoscopelus elongatus (Costa, 1844); Symbolophorus veranyi (Moreau, 1888) [Myctophidae]; Argyropelecus hemigymnus Cocco, 1829; Maurolicus muelleri (Gmelin, 1789) [Sternoptychidae]; Stomias boa (Risso, 1810); Chauliodus sloani Bloch et Schneider, 1801 [Stomiidae].

Figure 1. 

Sampling locations: North Aegean Trough (1), northern Euboean Gulf (2), Saronic Gulf (3), Cretan Sea (4), Gulf of Corinth (5).

Measured fish weight (W) [g] and length (TL) [cm] data were fitted to the power function

W = aTL^b

where a and b are the intercept and slope of the power equation, respectively. Data were transformed, using their natural logarithmic values and adjusted to a linear regression model by application of the least squares method, as to estimate length–weight parameters a, b (Kuriakose 2017). Intercept values (parameter a) give an indication of the expected weight at 1 cm of length for each species (Olivar et al. 2013). Confidence intervals (CI) of the parameters were calculated at the 95% confidence level and the resulting parameter b was evaluated, using a Student’s t-test (López-Pérez et al. 2020), as to inspect whether or not the sampled populations’ divergence from isometric growth (b = 3) was statistically significant, consequently indicating positive (b > 3) or negative (b < 3) allometry (Froese 2006).

The estimated values of the parameter b were compared to values reported for the same species in similar studies from the western Mediterranean (Olivar et al. 2013), the North (Fock and Ehrich 2010) and the tropical Atlantic (López-Pérez et al. 2020).

A total of 6214 fish individuals were used in the current length–weight analysis, but they were not equally distributed across species (Table 1). Fitted length–weight equations gave high coefficients of determination (r²) with values ranging from 0.943 for Argyropelecus hemigymnus to 0.983 for Diaphus metopoclampus. With the exception of Diaphus holti and Symbolophorus veranyi, the parameter b diverged significantly from isometry. Only two species (Benthosema glaciale and Chauliodus sloani) displayed negative allometry, while for the remaining 12 species a positive allometry (b > 3) was found, with the highest growth coefficient claimed by Stomias boa and Diaphus rafinesquii. The median value of parameter b for all species was 3.236 and 50% of its values ranged from 3.173–3.323.

Considerable variations of the parameter b between our findings and other studies from the Atlantic and the western Mediterranean were observed, especially compared to those from the North Atlantic (Table 2) (Fock and Ehrich 2010; Olivar et al. 2013; López-Pérez et al. 2020).

The current study attempted to assess length–weight equations for mesopelagic fish populations from the eastern Mediterranean Sea, setting the base for further biological studies necessary to support future management and research. Length–weight relations in the region have been examined in a localized context for only a few of the species considered here (e.g., Argyropelecus hemigymnus, Diaphus metopoclampus, Stomias boa; see Deval et al. 2014, Lampanyctus crocodilus, Chauliodus sloani; see Bayhan et al. 2020), while for others, to our knowledge, information is completely absent from the eastern basin or even from the entire Mediterranean (Diaphus rafinesquii). Length–weight parameters have been suggested to reflect environmental variations in species’ habitats, as well as adaptive mechanisms and intrinsic characteristics, which affect their ontogenetic development (Froese 2006; Eduardo et al. 2020b). In the presently reported study, fish samples derived from multiple seasons and across a wide geographical area encompassing open seas and enclosed gulfs and possibly including populations with indications of genetic differentiations (e.g., Hygophum benoiti) (see Sarropoulou et al. 2022); therefore, results can help derive conclusions for the estimated values of parameter b at the species level (Froese 2006).

For the majority of fishes studied herein, b was within the expected range of 2.5 and 3.5, (Froese 2006), although Stomias boa and Diaphus rafinesquii exhibited slightly higher values (3.52). The positive allometric growth observed in the majority of species is an indication of a more robust body growing faster in mass than in length, an attribute which may be essential for their diel vertical migrations (Olivar et al. 2013; López-Pérez et al. 2020). Contrarily, the negative allometric growth pattern, displayed here by two species, may be related to living in deep waters and to the absence of extended vertical migration (López-Pérez et al. 2020); this explanation seems plausible for Chauliodus sloani for which a temperature barrier inhibits its migration in warm regions (Eduardo et al. 2020a), but probably not for Benthosema glaciale, which shows a partial vertical migratory activity elsewhere (Dypvik et al. 2012), as well as in the study area (authors’ unpublished data).

Some intraspecific differentiations of the allometric coefficient among the current study and similar works were identified, which were more diverse compared to estimates from the North Atlantic. These can be attributed to fish growth affected by internal and external triggers (such as diet and habitat temperature) (Mazumder et al. 2016), to discrete population characteristics, but also to the sampled size ranges (Czudaj et al. 2022), and the type of length measurements (López-Pérez et al. 2020).

This work was part of the project MesoBED “Mesopelagic fish: Biology, ecological role and distribution of a disregarded trophic link”, funded by the Hellenic Foundation for Research and Innovation and the General Secretariat of Research and Innovation (Greece) (Project No. 449).