Prey composition of some common mesopelagic fish species from Hellenic Seas

Abstract

Although mesopelagic fishes have long been considered to be an important ecological link between zooplankton and top predators, constituting at the same time a significant factor in carbon transfer dynamics, studies of their feeding ecology have until now been scarce. In the present study we analyzed the stomach contents of some of the most commonly sampled mesopelagic fishes in the Greek Seas, during midwater trawl sampling surveys in November 2018 and in April and December 2019. Specifically, the stomach contents of 555 individuals belonging to eight mesopelagic species (six myctophids and two sternoptychids), sampled from the Corinthian Gulf (Ionian Sea - Central Mediterranean) and the Cretan Sea (East Mediterranean), were examined. Our analysis included the prey composition of species: Benthosema glaciale, Ceratoscopelus maderensis, Diaphus holti, Hygophum benoiti, Myctophum punctatum and Notoscopelus elongatus in family Myctophidae and Argyropelecus hemigymnus and Maurolicus muelleri in family Sternoptychidae. Stomach vacuity (VI%) was high for most myctophids (>50%), while sternoptychids’ stomachs were notably fuller during the time of sampling. Recorded individual prey items reached a total of 2426 in number. Myctophid’s C. maderensis stomachs had the highest average number of recorded prey items per examined stomach, followed by the sternoptychid Maurolicus muelleri. In terms of numerical abundance, frequency of occurrence and prey weight contribution, various Crustaceans were, in the majority of cases, the most important identified prey item of investigated mesopelagic predators’ diet and were mainly represented by calanoid copepods. Calanoids identified to species level included Calanus helgolandicus, Ctenocalanus vanus, Euchaeta marina and Paracalanus parvus. Other observed prey items were classified in various orders of the classes Malacostraca, Actinopterygii and Polychaeta. The present study intends to be the basis for future ecological assessments in the central and eastern parts of the Mediterranean, regarding the importance of mesopelagic ecosystems and their contribution to marine food webs.

Full text

Prey composition of some common mesopelagic fish species
from Hellenic Seas
Nicholas Badouvas1, Konstantinos Tsagarakis1, Stylianos Somarakis2, Paraskevi K. Karachle1
1. Hellenic Centre for Marine Research (HCMR), Institute of Marine Biological Resources and Inland Waters (IMBRIW), 46.7 km Athinon-Souniou, Anavyssos, Greece
2. Hellenic Centre for Marine Research (HCMR), Institute of Marine Biological Resources and Inland Waters (IMBRIW), Former American Base, Gournes, Heraklion, Crete
Introduction
Due to their high abundance and worldwide distribution, mesopelagic fishes have been regarded
as putting high preying pressure upon their zooplankton prey (Watanabe et al., 2002) with many of
their species exhibiting vertical diel migratory behaviour to the epipelagic layer, even reaching the
surface, in order to feed during the night (Hulley, 1990). In turn, they become prey for larger
pelagic fish species, cephalopods, seabirds and mammals (Valls et al., 2014). Although
mesopelagic fish species have long been considered to be an important ecological link between
zooplankton and top predators, studies of their feeding ecology in the Mediterranean have until
now been scarce. The present work intends to contribute to filling knowledge gaps on
Mediterranean mesopelagic fishes’ diets and to be a base for future ecological assessments.
Materials & Methods
We analyzed the stomach contents of some of the most commonly sampled mesopelagic fishes in
Greek Seas, during midwater trawl sampling surveys in November 2018 and in April and
December 2019. Specifically, the diet contents of 555 individuals belonging to eight mesopelagic
species (six myctophids and two sternoptychids), sampled from the Corinthian Gulf (Ionian Sea -
Central Mediterranean) and the Cretan Sea (East Mediterranean) (Fig. 1) were examined. Fish
samples were collected with the use of two types of pelagic trawls and a midwater frame onboard
R/V Philia. A pelagic trawl with a mouth opening of 12 x 7 m and a netting mesh of 16 mm was
employed as the primary fishing gear, while use of a smaller Sardonet trawl (3.7 x 2.1 m of mouth
opening and a netting mesh of 5mm) and of a Methot frame sampler (square mouth opening of
1.5 m with a netting mesh of 1mm) was complimentary.
Figure 1. Sampling station locations and characteristics in the Corinthian Gulf (upper map image) and the Cretan Sea
(lower image).
Our analysis included the prey composition of species: Benthosema glaciale (N = 48),
Ceratoscopelus maderensis (N =230), Diaphus holti (N =33), Hygophum benoiti (N = 6),
Myctophum punctatum (N =14)and Notoscopelus elongatus (N =53)in family Myctophidae and
Argyropelecus hemigymnus (N =79)and Maurolicus muelleri (N =92)in family Sternoptychidae.
Stomach vacuity (%VI)of examined specimens, as well as the numerical abundance (%N), weight
contribution (%W)and frequency of occurrence (%F)of identified prey were estimated and the
Index of Relative Importance (%IRI) was subsequently calculated.
References
1. Hulley, P.A., 1990. Myctophidae. In: Quero, J.C., Hureau, J.C., Karrer, C., Post, A., Saldanha, L. (Eds.), Check-list of the Fishes of the Eastern Tropical Atlantic, vol. 1. UNESCO, Paris, pp.398–467.
2. Valls, M., Olivar, M.P., De Puelles, M.F., Molí, B., Bernal, A. and Sweeting, C.J., 2014. Trophic structure of mesopelagic fishes in the western Mediterranean based on stable isotopes of carbon and nitrogen.
Journal of Marine Systems,138:160-170.
3. Watanabe, H., Kawaguchi, K. and Hayashi, A., 2002. Feeding habits of juvenile surface-migratory myctophid fishes (family Myctophidae) in the Kuroshio region of the western North Pacific. Marine Ecology
Progress Series,236:263-272.
XVII European Congress of Ichthyology
Results & Discussion
Recorded individual prey items in all sampled fishes’ stomachs reached a total of 2426 in number.
Stomach vacuity (%VI) was high for most myctophid species (>50%) from both areas, while the
two sternoptychids’ stomachs sampled in the Corinthian Gulf were notably fuller during the time of
sampling. C. maderensis’(Myctophidae) specimens claimed the highest average number of
recorded prey items per stomach, followed by M. muelleri’s (Sternoptychidae). In most cases,
various crustaceans were the most important identified prey in the investigated mesopelagic
fishes’ diets, so much in the Corinthian Gulf (Fig. 2 and Fig. 3), as in the Cretan Sea (Tab. 1).
Figure 2. Prey numerical abundance (%N)and frequency of occurrence (%F)in the diets of the eight mesopelagic
fish species sampled from the Corinthian Gulf.
Calanoids identified to species level in the stomach contents from the Corinthian Gulf samples
included Calanus helgolandicus,Ctenocalanus vanus,Euchaeta marina and Paracalanus vanus.
Other observed prey items were classified in the classes Malacostraca, Actinopterygii and
Polychaeta. Taxonomic resolution and quantity of prey items from the Cretan Sea samples were
lower due to the smaller number of examined specimens.
Figure 3. Index of Relative Importance (%IRI) of prey items identified in the investigated mesopelagic fishes’ diets
sampled from the Corinthian Gulf. B. glaciale’s identified prey items included exclusively undefined crustaceans.
Table 1. Prey composition of investigated mesopelagic fish species’ diets sampled from the Cretan Sea (TL: total
length, N: number of specimens).
More information on mesopelagic fishes’ diet habits in different parts of the Central and Eastern
Mediterranean will prove necessary in the assessment and the parametrization of marine food
webs, as well as in quantifying the resulting energy transfer to top predators of commercial or
conservation interest.
Cretan Sea
Benthosema glaciale Ceratoscopelus maderensis Diaphus holti Notoscopelus elongatus
Prey items %N %F %W %IRI %N %F %W %IRI %N %F %W %IRI %N %F %W %IRI
Crustacea (Total) 22,22 30,77 30,77
100,00
100,00
100,00
100,00
50,00 50,00 57,14 46,15 70,45
undefined Crustacea 22,22 30,77 30,77 28,65 50,00
100,00
50,00
100,00
50,00 50,00
100,00
57,14 46,15 70,45 97,60
Copepoda
100,00
50,00 50,00
Fish 55,56 38,46 50,00 71,35 7,69 2,27 0,29
Polychaeta 14,29 7,69 2,27 2,11
Particulate Organic Matter 22,22 30,77 19,23 50,00 50,00 28,57 38,46 25,00
N32 20 25 27
TL range 26 -70 37 -70 42 -68 65 -123
%VI 63% 85% 92% 52%
Prey item N 9 3 3 7