울릉도와 독도에 출현하는 띠볼락(Sebastes zonatus Chen and Barsukov, 1976)과 말쥐치(Thamnaconus modestus(Günther, 1877))의 생물역학적 관계와 계절적 비만도지수의 첫 보고

First Biometric Relationship and Seasonal Condition Factors of Sebastes zonatus Chen and Barsukov, 1976 and Thamnaconus modestus (Günther, 1877) Inhabiting the Waters of Ulleung-do and Dokdo

Abstract

본 연구는 울릉도와 독도 해역에 서식하는 띠볼락(Sebastes zonatus Chen and Barsukov, 1976)과 말쥐치(Thamnaconus modestus (Günther, 1877))의 체장-체중 관계 (LWR)와 계절적 비만도지수 (K) 정보를 국내 최초로 보고하였다. 두 종의 체장-체중 관계는 모든 계절에 높은 상관성을 보였으며(r²>0.959), LWR 회귀 기울기는 두 종 모두 저수온기와 고수온기 사이에 유의하게 차이가 있었다. 두 종의 비만도지수는 저수온기에 고수온기보다 현저히 높아 산란기와 관련된 비만도 증가를 반영하였다. 본 연구의 결과는 띠볼락과 말쥐치의 생물학적 특성을 이해하고 향후 두 어종에 대한 보존 및 관리 계획 개발에 유용한 자료를 제공할 것이다.

This study is the first to report the biometric information between the length and weight relationships (LWR) and seasonal body condition factors (K) of Sebastes zonatus Chen & Barsukov, 1976 and Thamnaconus modestus (Günther, 1877) inhabiting the waters off Ulleung-do and Dokdo. The LWRs in spring and summer, and all seasons combined were highly correlated (r²>0.959), and the regression slopes of LWRs were significantly different between the spring and summer in both species. The body conditions of the two fish were significantly higher during the spring than during the summer, reflecting their fatness in relation to spawning. The results from this study contribute to the understanding of the biology of S. zonatus and T. modestus and provide useful data for the development of conservation and management plans for these species.

INTRODUCTION

Body morphometric data of fishes are useful tools in fisheries sciences because fish species (intraspecific units) and populations (stocks) often have unique morphological characteristics that can be identified and compared via morphometric relationships (King, 1995; Froese, 1998; Park et al., 2018). Among the morphometric data, the exponential parameter b and intercept a of length-weight relationships (LWRs) are applicable for assessing fish stocks and populations (Ricker, 1968), because stock assessment models require information about the body weight (e.g., biomass) of populations to estimate and regulate optimal catches of fish stocks (Breen et al., 2003; Froese et al., 2014). Morphometric relationships (e.g., length-weight and length-length relationships) also can be used for comparing relative growths between taxonomically similar fish species and/or estimating alternative growth patterns of understudied fish species (Kwak and Park, 2016; Park et al., 2017). Fulton’s condition factor is widely used to measure the health of overall fish body and assess seasonal patterns in the body condition of the individual fish (Le Cren, 1951; Froese, 2006) because heavier fish of a given length are in better condition than lighter fish (Bagenal and Tesch, 1978).

Sebastes zonatus and Thamnaconus modestus are ree-fassociated subtropical and/or temperate marine fish species inhabiting shallow coastal regions (Nakabo, 2002; Kim et al., 2005), and belong to the families Sebastidae (rockfishes) and Monacanthidae (filefishes), respectively. They are mainly distributed in the Northwest Pacific region, especially the seas surrounding the Korean Peninsula and Japanese islands, as well as the East China Sea (Froese and Pauly, 2022). Sebastes zonatus was previously regarded as a color scheme for S. vulpes but was differentiated by Chen and Barsukov (1976). However, because of its similar shape to S. vulpes, the species is still regarded as a color variation and is often misclassified as S. vulpes. This species is only abundant around Ulleung-do and Dokdo coastal regions (Chung et al., 2015), but rarely occurred in other coastal habitats of Korean Peninsula.

Thamnaconus modestus is one of the major commercial fish species harvested from coastal fisheries in the southern and southeastern seas of South Korea. The commercial landings of T. modestus have gradually decreased since the 1980s due to several reasons including overfishing (Kim et al., 2016). However, a lack of fisheries data for these species makes difficult the formulation of management strategies. Several studies have documented the biology and ecology of the species globally (e.g., Baeck et al., 2012b; Kim et al., 2016; Choi et al., 2020; Goto and Takanashi, 2020), but there is a lack of information regarding the biometric relationships and seasonal body condition factors of the two species. Only a few studies reported their age and growth (Kim et al., 2016; Goto and Takanashi, 2020).

This study provides the first biometric information (e.g., LWR) and seasonal body conditions of S. zonatus and T. modestus inhabiting the waters off Ulleung-do and Dokdo. Moreover, this study will serve as baseline data to support future studies on the life history and evaluate the status of fisheries resources of these species.

MATERIALS AND METHODS

1. Sample collection

Sampling was conducted in the waters off Ulleung-do and Dokdo, Korea (37°28′N, 130°54′E and 37°14′N, 131°51′E, respectively). Fish samples of bycatch were collected during the spring (April) and summer (August) over three consecutive years (2020~2022) at depths between 30 m and 140 m using a bottom gillnet (75 m length, 2 m height, and 90 mm mesh). The individuals were snap-frozen and transported to the laboratory immediately after the collection. For each specimen, total length (TL) and wet body weight (BW) were measured to the nearest millimeter and gram, respectively.

2. Data analysis

For each species, the allometric equation of the length-weight function, BW=a TL^b, was fitted to the data using linear regressions of log10-transformed data, where a and b are the intercept and allometric coefficient, respectively (Brodziak, 2012). The standard errors (SE) of parameters a and b and the statistical significance level of r² for the LWRs were estimated. Extreme outliers were removed before fitting the linear regression according to the suggestion of Froese et al. (2011). The LWRs values were summarized for each season (spring and summer) and all seasons combined. Differences in b values from the isometric value of the three were tested using the Student’s t-test. A full factorial analysis of covariance (ANCOVA) was used to test the effect of the categorical factor of seasons on the relationship between body weight and total length.

Fulton’s condition factor (K) was calculated for each fish using the equation K=100×BW/TL³ (Pauly, 1984; Froese, 2006). Differences in body condition were examined with respect to the season. Since there was no evidence of heteroscedasticity (P>0.05), a Student’s t-test was used to assess if the b values were significantly different from isometric growth (b=3), as well as differences in body condition factor between spring and summer. All statistical analyses were performed using the SYSTAT software (Systat version 18.0, SPSS Inc., Chicago, IL, USA). An assumed significance level of 0.05 was used in all statistical analyses.

RESULTS

Length-weight regressions were performed on 109 S. zonatus specimens and 236 T. modestus specimens. The estimated parameters of LWRs and their descriptive statistics by season and all seasons combined are provided in Table 1. All LWRs were highly significant (P<0.05), with r² values ranging from 0.985 to 0.990 for S. zonatus and 0.959 to 968 for T. modestus. Estimated b values ranged from 2.997 during the summer to 3.194 during the spring for S. zonatus, while it was between 2.937 (summer) and 3.072 (spring) for T. modestus. Overall, the b value of the LWRs was higher than 3 (Student’s t-test, P<0.05) for S. zonatus exhibiting positive allometric growth, while the b value of T. modestus was lower than 3 (P<0.05), exhibiting negative allometric growth. Seasonally, the b values did not differ significantly from 3 (P>0.05) for S. zonatus in the summer and T. modestus in the spring, showing isometric growth (Table 1). ANCOVA results revealed that the slopes (allometric coefficient) of the LWRs were significantly different between the spring and summer for both S. zonatus and T. modestus (Fig. 1; P<0.05).

Table 1. Length-weight relationship (LWR) parameters of Sebastes zonatus and Thamnaconus modestus inhabiting the waters off Ulleung-do and Dokdo

Fig. 1. Power fitted curve of each length-weigh relationship for all specimens during spring and summer of Sebastes zonatus (A) and Thamnaconus modestus (B).

The mean Fulton’s body condition of S. zonatus and T. modestus ranged from 1.196 to 2.495 (mean±SD=1.869±0.211) and 1.051 to 1.738 (mean±SD=1.358±0.130), respectively. The body conditions were significantly different between the spring and summer for both species, showing consistently higher values during the spring (Student’s t-test, both P<0.05; Fig. 2).

Fig. 2. Box plots of condition factors for Sebastes zonatus (A) and Thamnaconus modestus (B) during spring and summer, and all seasons combined. Asterisk indicates significant differences at P=0.05.

DISCUSSION

This study provides the first report of LWR for S. zonatus and T. modestus. The estimated b values of LWRs from this study were within the range of 2.5 and 3.5, which was observed for most fish species (Froese, 2006). S. zonatus and T. modestus showed different growth patterns wherein the b values of the two species were significantly different from that of the isometric value of ‘3’ (i.e., overall, 3.133 vs. 2.876), with positive or negative allometric growth patterns, respectively (Froese, 2006). Positive and negative allometric growths indicate that the fishes become “heavier or lighter for its length” as they grow larger, respectively (Ricker, 1968). Therefore, S. zonatus gains more weight than its length, while T. modestus grow more in length than in weight. In addition, results for intercepts (a) of LWRs indicate that the body shape of S. zonatus was close to ‘Fusiform’ while T. modestus was close to the ‘Short and deep’ categories (Froese, 2006).

The regression models estimated the maximum length measurements of the fish beyond those previously recorded. The maximum size of the S. zonatus and T. modestus (42.5 cm TL and 42.6 cm TL, respectively) collected in this study were comparable to that of the previous records in FishBase (Froese and Pauly, 2022) and other literature (e.g., Kim et al., 2016; Goto and Takanashi, 2020). The maximum length of S. zonatus was 37.0 cm SL in FishBase, and 35.9 cm SL by Goto and Takanashi (2020).

As the former studies did not provide the total length of S. zonatus, we estimated it using length-length regression of morphologically similar S. vulpes (Baeck et al., 2012a). As a result, the estimated maximum total length of S. zonatus is approximately between 42.0 and 44.0 cm, which is similar to or slightly higher than that recorded in this study. The maximum length of T. modestus was recorded as 36.1 cm TL in FishBase, and Kim et al. (2016) recently reported it as 42.1 cm TL. Therefore, this is the first study that reports the maximum lengths for T. modestus and provides new records of the two species to update size information in FishBase, a global biological database of fish species.

The allometric coefficients (b values) of LWR of S. zonatus and T. modestus were significantly higher during the spring. Higher b values indicate a tendency towards positive allometric growth, which implies an increase in either body condition, height, and width rather than in length (Froese, 2006). Moutopoulos et al. (2013) reported significantly higher b values in the LWRs of several fish species during the spawning period. This might be due to the presence of a large number of mature individuals(large-sized specimens) with higher rates of increase in BW than in body length (TL) during spawning periods (Hossain et al., 2013).

Additionally, better body conditions were recorded during the spring. Seasonal changes in body conditions are usually driven by food availability, environmental conditions, and reproductive status (Weatherley and Gill, 1987; Chellappa et al., 1995). In many fishes, Fulton’s condition factor differs seasonally because of fluctuations in food supply and consequent changes in food reserves of fish (Hossain, 2010; Lavergne et al., 2013) as well as a reproduction (Brosset et al., 2017). Vazzoler and Vazzoler(1965) stated that the condition factor does not merely reflect the feeding condition of the adult stage, but also includes the state of gonadal development based on the consumption of fat reserves during the spawning period. An increase in the condition towards the spawning season and a sharp decline after spawning are general patterns in adult fish (Le Cren, 1951). On the other hand, a decrease in body condition factors during spawning periods reflects changes in feeding patterns, which could be a behavioral response to certain stressors (Brown et al., 1987), such as guarding eggs and offspring in some goby species (Colombini et al., 1996; Baeck and Park, 2015). The spawning seasons of S. zonatus and T. modestus occur during spring, which coincides with our colder sampling season (Kim et al., 2016; Choi et al., 2020; Goto and Takanashi, 2020). Therefore, such seasonal changes in LWR parameters and condition factors may reflect spawning-related variations in body conditions for both species.

CONCLUSION

This study provides the first biometric data on LWRs and seasonal body condition factors of S. zonatus and T. modestes, which are abundant in coastal fish assemblages in both Ulleung-do and Dokdo. These results contribute to future conservation studies of the species and could be useful information for fishery biologists seeking management plans for the species in Korea.