Studies on the biology of non-commercial species, based on the example of the fourbeard rockling Enchelyopus cimbrius (L., 1766) (Gadiformes: Lotidae) in the southern Baltic

The paper includes a detailed analysis of selected elements of the biology of the forebeard rockling. The study was conducted in the southern Baltic Sea. The sex structure, fertility, stomach contents, age, and growth of the species were analysed. As a result of the study, double predominance in numbers of males over females and high diversity in terms of development and size of oocytes were determined, suggesting partial spawning. A strong correlation was recorded between fertility and body mass and length. The analysis of the content of the ish stomachs revealed dominance of Polychaeta, Crustacea, and ish. Analyses of age and growth were also conducted, based on the otoliths. Individuals of 4 to 13 years of age were found. In the samples analysed, males were older than females. Additionally, differences in growth between the samples analysed were determined, along with similarities in growth between ish collected from the southern Baltic and the Atlantic forebeard rockling.


INTRODUCTION
Knowledge on biology is a determinant of any activities aimed at studies on selected species of ichtiofauna. Recording the occurrence of a given organism only contributes to the determination of its geographical distribution. Although organisms of one species prefer similar abiotic and biotic conditions, different geographical occurrence always entails certain variations. Those may concern the body size, age and growth rate, fertility, or alimentation. Such differences are the easiest to observe for commercial ish. Non-commercial ish species are equally important, however, because they constitute an inherent part of the aquatic ecosystem, allowing for the energy low through all its trophic levels (Holmlund and Hammer 1999). One of such ish species is the fourbeard rockling Enchelyopus cimbrius (L.). In spite of its quite widespread occurrence in waters from North America to Europe, it is still a poorly known species. The main reason for the insuficiency of information on its biology is lack of commercial interest in the ish. The fourbeard rockling belongs to family Lotidae, living and reproducing in the Baltic Sea (Heese 1998; Krzykawski et al. 1999; Więcaszek et al. 2015).
An insight into selected elements of the biology of the fourbeard rockling is important from the point of view of the functioning of the Baltic Sea and conducting sustainable ishery management. Therefore, the objective of this paper is to determine the sex and age structure, growth rate, and total fertility of the fourbeard rockling from the Southern Baltic Sea, as well as the qualitative and quantitative composition of contents of its digestive system.

MATERIALS AND METHODS
The study material was collected in the years 1995 and 2000-2001 by means of a seabed trawl in the southern part of the Baltic Sea (Fig. 1). Six fourbeard rockling samples were analysed ( Table  1).
Fertility of the fourbeard rockling was investigated by extracting oocytes from spawn samples collected on 11 and 27 April 2001. Sixty gonads were analysed. Each gonad was weighed. A small portion of oocytes was then sampled and weighed with accuracy to 1 mg. The oocytes were placed on a Precoptic Co glass tray with a 0.5 mm mesh net. They were distributed onto the entire surface of the glass by illing the chamber with 1 ml of water. Next, images were taken by means of a stereoscopic microscope and digital camera. The images were sent to a computer, where the oocytes were counted and their diameters measured with the application of the Lucia software. The results obtained were used to calculate total fecundity and to determine the size distribution of individual oocytes according to their length.
For the purpose of examining the stomach contents, the stomachs together with its content were removed, marked, and placed in 4% formaldehyde. A taxonomic analysis of foraged organisms MAGDALENA LAMPART-KAŁUżNIACKA, TOMASz HEESE Pobrane z czasopisma Annales C -Biologia http://biologia.annales.umcs.pl Data: 14/09/2023 17:45:40 U M C S 45 was conducted by means of a stereoscopic microscope. Determined specimens were counted and weighed. A total of 488 stomachs extracted from fourbeard rockling samples (K2,K3,K4,K5,Ł) were examined. The data obtained were used to calculate the weight participation (i.e. the percentage share of the weight of individual components in the total weight of food and frequency of occurrence of a given food component, determining the number of ish in which the food component was found (10; 21).
In order to analyse the age and growth each ish was measured with accuracy to 1 mm. Then, the otolith was extracted by making an anatomic incision on the head. A total of 105 of the structures were used for the analysis. They were all submerged in resin solution and cut by means of an Isomet TM low speed saw with a diamond blade disk. The fragments obtained were used to prepare microscope slides including the otolith nucleus. Photographic documentation was prepared using a Nikon Eclipse E 400 microscope and Nikon Coolpix 950 digital camera. The images revealed the otolith structure with marked opaque and haline zones. The zones were counted by means of the Lucia software (licence No. 11785). Distances from the centre to the edges of individual zones were also measured. Following the methodology presented in papers by Secor et al. (19), Gabriel et al. (5), and Panili et al. (15), each pair of the rings was assumed to constitute annual growth. Data from back readings were used to calculate the growth rate in consecutive years of age of the fourbeard rockling. For this purpose, the power function was used, describing a curvilinear dependence between the body length (L) and otolith radius (O) (6) L=k*O n where: L -total length of a ish at the moment of catching, O -total otolith radius at the moment of catching, k -angular coeficient, n -index of the power. Next, the corrective adjustment was calculated for total otolith radius, according to the following formula: f=O t /O c where: f -corrective adjustment O t -theoretical otolith radius calculated following the formula for total ish length, O c -total otolith radius at the moment of catching a given ish. In order to calculate the growth of a given ish in consecutive years of age, the following formula was applied: L 1 =k*(fO 1 ) n L 2 =k*(fO 2 ) n L n =k*(fO n ) n , where: For research on fourbeard rockling population structure, 1011 ish from six isheries of the southern Baltic Sea were used (Fig.1). In majority of samples, twice as many males (n = 638) as females (n=373) were found. Only in K5, the opposite situation was recorded (Table 1).  Total fertility was estimated based on female gonads from Kołobrzeg isheries K4 and K5. The samples were taken in April 2001, i.e. at the time considered as the fourbeard rockling spawning season. A total of 60 gonads were analysed, 30 of which belonged to sample K4, and 30 to sample K5.
High diversity in the degree of development of oocytes was determined in both samples analysed. The gonads included eggs with a diameter of 0.1-0.92 mm. The most numerous were oocytes in the fourth size class, with a diameter of 0.28-0.33 mm. Females from K5 ishing ground had somewhat higher total fertility. Nonetheless, in the gonads analysed, oocytes of the 3 rd and 4 th size class with inconsiderable diameters were usually found (Fig. 2).
Linear functions were also determined, and correlation coeficient values were calculated for the relation of fertility to body mass (Fig. 3a), and fertility to body length. A stronger correlation was determined for the former relation analysed, where r>0.9 in both samples. Simultaneously, the correlation coeficient value (r>0.7) suggests a signiicant correlation, also for the relation of fertility to body length (Fig. 3b). The analysis of the contents of the stomach contents was carried out on samples K2, K3, K4, K5 and Ł. Occurrence frequency and weight proportions of individual food components were calculated The main source of alimentation for the fourbeard rockling is macrozoobenthos of the southern Baltic Sea. Apart from bottom invertebrates, the stomach contents sporadically included ish, signiicantly increasing the weight proportion. Among benthic organisms, numerous polychaetes and crustaceans were found. As a result of the analysis of the weight proportion and occurrence frequency index of individual macrozoobenthos taxa, predominance of mysid shrimp Mysis mixta was determined in sample Ł. In stomach contents of ish from Kołobrzeg isheries, polychaete Harmothoe sarsi prevailed. In sample K4, quite numerous Corophium volutator was recorded, and in K4 and K5 -brown shrimp Crangon crangon, Pontoporeia sp., Halicryptus spinulosus, and Nephthys ciliata ( Table 2). Notice that for ish from isheries K4 and K5, a signiicant weight proportion among the taxa mentioned was that of Nephthys ciliata, and among not mentioned taxa -that of ish. The samples also included a large weight proportion of unidentiied contents ( Table 2). The age of the fourbeard rockling was estimated based on otolith microscope slides subject to digital image analysis. The age of ish analysed varied between 4 and 13 years The youngest (4-year-old) individual was recorded in the sample Ł. The oldest (13-year-old) ish were found in two samples, K5 and Ł. The most numerous were seven-year-old specimens for ish from areas K3 and Ł, and ten-yearold specimens from areas K1 and K5 (Tables 3, 4, 5). In the samples analysed, namely K1, K3, and Ł, the estimated age of males was higher than that of females. Only in sample K5, the situation was opposite (Tables 3, 4, 5).
gROwTh Total length growth of fourbeard rockling from the southern isheries of the Baltic Sea was estimated with the application of back readings. Empirical data (n = 103) were used to calculate the constants of the power function.
The obtained value of 0.7169 was substituted into the power BPH equation.
The length of the fourbeard rockling grew the fastest in its irst year of age. From the second year, the total length growth decreased successively. Variability in the individual body length growth was also determined in the irst year. For majority of samples analysed, the variability coeficient reached higher values in the initial period of life than in consecutive years (Tables 3, 4, 5).
As a result of the analysis of the growth in four fourbeard rockling samples, differences in total length growth were determined. Fish from area K1 grew the fastest, the total length growth was somewhat slower for samples K3 and K5, and ish from Łeba (Ł) had the slowest growth. The differences were revealed by comparison of entire samples and sex. For males from Kołobrzeg ishing grounds, very similar body length growth was determined, which was higher than that of males from Łeba ishing ground (Table 3, 4, 5).

DISCUSSION
The fourbeard rockling is a demersal ish (9) occurring at depth range of 20-250 m (16). It is caught at the western continental margins of the Atlantic Ocean at a depth of 170-227 m (4). In the Baltic Sea, the fourbeard rockling occurs at much smaller depths, with a maximum of 150 m (18). In the scope of this research, it was caught at a depth of 40-93 m. The fourbeard rockling grows up to a length of 50 cm (13). The total length of the fourbeard rockling from the Baltic Sea varied between 18.3 and 37.8 cm, and the length range determined for the Atlantic ish was 9.5-32.8 cm (4). The largest and the smallest specimens were recorded in the sample from Łeba (Ł). The most numerous, however, were individuals of 20-40 cm (2; 14). This was also conirmed by this research, in which ish with a length from 20 to 30 cm prevailed.
The determination of the sex structure and percentage share for the analysed samples of the fourbeard rockling from the Baltic Sea revealed a signiicant prevalence of males over females. Twice as many males were recorded in ive samples analysed. Only in population K5, the situation was opposite. Similar results were obtained by Deree (4) for the Atlantic population, also dominated by males in quantitative terms. Such a sex distribution, however, is typical of older ish, with a total length of more than 25 cm. In lower length classes of the Atlantic forebeard rockling, Deree (4) observed equal participations of both sexes in the population structure, or even an insigniicant predomination of females. The opposite sex structure in sample K5 may result from forming spawning stocks by females. The sample was collected in April at the maximum depth recorded in the research (81-93 m), and according to Wheeler (24), spawning of the species occurs at larger depths between February and August (14).
The fourbeard rockling is a ish living in shoals and preferring sandy or muddy bottom (26). It feeds on benthos and small ish (19). The analysis of the stomach contents of populations from the Baltic Sea conirmed the literature data determining macrozoobenthos as the main component of food of the forebeard rockling. Certain differences were observed, however, in dominance in terms of numbers and weight of identiied taxa, depending on the place of collecting a given sample. In populations from Kołobrzeg, the most numerous were polychaetes, namely Harmothoe sarsi and Nephthys ciliata, and in the sample from Łeba -crustacean mysid shrimp Mysis mixta. Studies by Deree (4) concerning stomach contents of the Atlantic population of the fourbeard rockling also conirm a considerable participation of invertebrate organisms (Polychaeta, Decapoda) in the food of particularly older specimens. The author of the cited paper also states that younger ish prefer molluscs and pelagic and pelagic-benthic invertebrate forms. Studies on stomach contents of larvae and juvenile forms of the fourbeard rockling were also conducted by Tully and O'Ceidigh, (22) and Albert (1). The studies revealed the occurrence of pelagic invertebrates in stomachs of the forebeard rockling. The presence of pelagic and pelagic-benthic organisms in the stomach contents conirms the earlier supposition on periodical abiding of the ish in pelagic waters (1: 22). Older ish prefer bottom waters. Their stomach contents are dominated by typical bottom fauna, as evidenced by both this study and the paper by Deree (4).
The present paper attempts to estimate total fertility of the forebeard rockling. Two samples were used, collected in April 2001 at a depth of 50-64 m and 81-93 m, respectively. The fourbeard rockling is characterised by relatively high fertility (450,000 and 470,000 eggs, respectively). The fertility calculated is approximate to the literature data, where it is estimated at the level of 500,000 eggs (2; 27).
The degree of development of oocytes, determined in the scope of the research, and their size variation for females analysed (0.1-0.9 mm) suggests partial spawning. A higher participation of eggs with a smaller diameter was also observed in a sample collected at the end of April. A female probably spawns several times. In the available literature two spawning terms are recognised: the irst one in the early months of spring, and the second in summer (16; 17; 18) states that the largest oocytes of Gadus morhua are laid during the irst spawn, while the smallest -with the last portion. This may be related to a certain reproductive strategy, where part of larvae from the later hatch falls prey to the faster growing and stronger fry of the earlier hatch The age of the fourbeard rockling from the Baltic Sea, estimated based on otoliths, varied between 4 and 13 years. The oldest ish were recorded in samples K5 and Ł. Their estimated age was higher than that determined by Deree (4) for the West Atlantic population of the forebeard rockling, and by Cohen et al. (3) for the East Atlantic population, where the authors recorded specimens with a maximum of 9 years of age. It is worth of notice that ish analysed in studies by Deree (4) (4) allowed for comparing the growth rate of the Atlantic fourbeard rockling with that of ish used for purposes of this paper. The analysis of graphs reveals that the Atlantic fourbeard rockling is similar in terms of growth rate to ish from Kołobrzeg, particularly K1 (Fig.4a). It is visible in the case of comparison of the entire samples and in comparison of females (Fig. 4b) that males of the Atlantic population also have the same growth rate as males from Kołobrzeg isheries (Fig. 4c). Fish from Łeba isheries (Ł) have a signiicantly lower growth rate.
A similar, almost identical growth rate of the Atlantic fourbeard rockling and ish from Kołobrzeg ishing ground suggests high similarity of both populations. Perhaps the populations of ish from the Baltic Sea have sporadic contact with the populations of the North Sea. Also differences in the growth rate revealed among groups analysed (ish from Kołobrzeg in relation to ish from Łeba ishing grounds) suggest the existence of separate stocks of the fourbeard rockling inhabiting the southern waters of the Baltic Sea. Both hypotheses could be conirmed by genetic analyses, which would contribute to a deeper insight into this interesting, widespread, but still little known ish species.
According to Holmlund and Hammer (8) and Kjell (11), recognition of the signiicance of studies on the biology of non-commercial species is necessary for obtaining greater knowledge on the structure and functioning of aquatic ecosystems. The ish constitute their integral part. Lack of basic information on the age, sex structure, and fertility of speciic populations considerably constrains