The length–weight relationship (LWR), condition factor (K), and biometric characteristics (body wall, viscera, and gonads) of the commercially important sea cucumber, Holothuria tubulosa Gmelin, 1791, from the northeastern Aegean Sea in Türkiye were analyzed based on 327 individuals collected monthly from October 2018 to September 2019. The total length (TL), wet weight (WW), gutted weight (BW), viscera weight (VW), and gonad weight (GW) were determined for each sea cucumber to the nearest 0.1 cm for length and 0.01 g for weight. The sex of each individual was identified through macroscopic observation of gonad coloration, facilitating the categorization of biometric relationships according to sex. Results revealed negative allometric growth (b = 1.458) for H. tubulosa, with length increasing more than weight. The allometric growth values, calculated as 1.208 in males and 1.584 in females, did not differ significantly between the sexes. The mean condition factor was 2.42 ± 1.04, displaying significant seasonal patterns, with peaks in February and October and the lowest values during summer. Notably, smaller individuals (<100 g) exhibited significantly higher K values, suggesting better health. A strong positive linear relationship was observed between wet and gutted weights (R² = 0.549). Body composition analyses revealed a declining proportion of gutted weight with increasing total wet weight, whereas the relative proportions of viscera, gonads, and water content were positively correlated with body size. These findings offer region- and species-specific insights that can enhance the accuracy of stock assessments and support adaptive management strategies for the H. tubulosa fishery.

Aegean Sea, condition factor, Holothuria tubulosa, length–weight relationship, stock assessment

Sea cucumbers (Holothuroidea) are ecologically and economically significant benthic invertebrates that play vital roles in nutrient cycling and sediment bioturbation in marine ecosystems (MacTavish et al. 2012; Purcell et al. 2016; Pasquini et al. 2023). In recent decades, their high commercial value, particularly in Asian markets, has led to intense harvesting pressure, raising concerns regarding the sustainability of wild populations (Rahman and Yusoff 2017; Ciriminna et al. 2024). One of the primary challenges in managing sea cucumber fisheries is the lack of comprehensive biological information on many new target species. Key biometric data, such as length–weight relationships (LWRs) and condition factors, are essential for robust stock assessments and the design of effective conservation and harvest strategies. The unique morphological plasticity of sea cucumbers, characterized by their soft bodies and variable water retention, complicates accurate measurement and consistent data collection, often leading to different measurement approaches (e.g., gutted, drained, or wet weights) across studies (Prescott et al. 2015; Harper et al. 2020).

LWRs provide a quantitative link between an organism’s linear dimensions and its biomass, offering insights into growth patterns, physiological condition, and energy allocation. In fisheries science, the LWR is widely used to estimate biomass from length-frequency data, evaluate stock health, and model population dynamics. The size composition of harvested individuals, along with the corresponding LWR parameters, is crucial for assessing the spatial and temporal impacts of exploitation on the population structure (Montgomery 1995; King 2007; Natan et al. 2015; Ahmed et al. 2018).

A key advantage of the LWR is its sensitivity to ecological and physiological variations influenced by factors such as sex, reproductive stage, habitat characteristics, and seasonal changes. Combined with other biometric indicators, LWRs are instrumental in calculating the condition factor, which reflects the nutritional status of individuals and the overall health of their environment (Natan et al. 2015; Aydın 2016; Ram et al. 2016). Reliable LWR models are essential for accurate conversions between morphometric traits and for robust stock assessments, particularly for species such as sea cucumbers, which are typically harvested by weight but monitored by length during field surveys. Integrating biological variability into LWR models enhances the precision of population estimates and supports the development of adaptive management strategies. These assessments support decision-making through predictive modeling and the implementation of harvest control rules aimed at ensuring long-term sustainability (Caddy 2004; Léopold et al. 2013; Wynsberge et al. 2013).

The Mediterranean sea cucumber, Holothuria tubulosa Gmelin, 1791, is one of the most commercially important temperate species along the Mediterranean coastal regions, particularly Türkiye’s northeastern coasts in the Aegean Sea, and constitutes a substantial share of the total sea cucumber harvest in the region. This species emerged as a major target for commercial sea cucumber fishing due to increased global market demand that began in the late 1990s (González-Wangüemert et al. 2015; Dereli and Aydın 2021). The increasing export volumes, particularly between 2012 and 2020, indicate high harvest levels of H. tubulosa in Türkiye, raising concerns about the risk of overexploitation of natural stocks (Aydın et al. 2023). In response, the Turkish Ministry of Agriculture initiated a quota-based management system for harvest quantities as of November 2020 to promote the sustainability of wild populations. Despite its ecological and economic importance, biometric data from natural populations prior to the quota system are scarce, with the most recent dataset dating back to December 2018 (Aydın 2020). However, region-specific and species-calibrated LWR models are essential for developing effective regulatory frameworks to ensure the sustainability of natural sea cucumber resources, particularly in the northern Aegean Sea, which has become a focal area for sea cucumber fisheries.

The main objective of this study was to investigate the length–weight relationship (LWR) and condition factor of H. tubulosa along the northeastern coast of the Aegean Sea, with a specific focus on seasonal and sex-related variations. Furthermore, this study sought to establish a robust, species- and region-specific LWR model. Considering the commercial significance of H. tubulosa, additional analyses were conducted to evaluate the relationships between key morphological parameters, such as wet weight, gutted weight, and viscera weight, across different size classes to provide predictive insights for sustainable stock management.

This study was conducted along the coastal waters surrounding the islands near Ayvalık District, located on the northeastern coast of the Aegean Sea, Türkiye (39°19.77′N, 026°36.55′E) (Fig. 1). This region is widely regarded by local fishers as one of the most productive harvesting grounds for commercial sea cucumber species in the Mediterranean Sea. Holothuria tubulosa samples were collected monthly from October 2018 to September 2019 from the shallow sublittoral zones, typically between 2 and 10 m in depth. A total of 327 individuals of various sizes were hand-picked by divers and subsequently transported to the laboratory in seawater tanks for further analysis.

Figure 1. 

Map showing the Holothuria tubulosa sampling area at the coasts of islands near the Ayvalık district located in the northeastern Aegean Sea, Türkiye (39°19.77′N, 026°36.55′E).

In the laboratory, each specimen was removed from the seawater tank, gently blotted with a paper towel to eliminate surface moisture, and then weighed using a precision balance with an accuracy of 0.01 g to determine the wet weight (WW). The total length (TL) was recorded from the mouth to the anus using a ruler, accurate to the nearest 0.1 cm. Ventral dissection was performed to remove the internal organs, including the intestines, respiratory trees, and reproductive system. The remaining body wall was dried with blotting paper and weighed to determine the gutted weight (BW). The gonads were carefully separated from the viscera, blotted, and weighed to obtain the gonad weight (GW). Sex was determined macroscopically based on gonad coloration: pale white gonads indicated males (M), whereas pink to orange gonads indicated females (F). Specimens lacking gonads or exhibiting indistinct coloration were classified as indeterminate (U). The remaining internal organs were blotted and weighed to determine the viscera weight (VW).

The length–weight relationship (LWR) was modeled using Pauly’s (1984) power function model, expressed as:

$$\mathrm{WW}=a{L}^b$$ (1)

where WW is the wet weight, L is the total length, a is the scaling coefficient, and b is the allometric exponent.

The b value was evaluated using a t-test to determine whether it significantly differed from 3. A value of b = 3 indicates isometric growth, whereas b ≠ 3 denotes allometric growth—positive when b > 3 and negative when b < 3 (Froese 2006).

Prior to analysis, records containing missing values or biologically implausible measurements were excluded to ensure data integrity. Separate LWR models were fitted for each month and sex category to assess temporal and biological variations.

Fulton’s condition factor (K) was calculated to assess the general health and robustness of individual H. tubulosa specimens. This index provides a standardized measure of body condition by relating wet weight to total length and was analyzed according to Pauly (1984):

$$K=\left(\frac{\mathrm{WW}}{L^3}\right)\times 100$$ (2)

where K is Fulton’s condition factor, WW is the wet weight of the individual [g], and L is the total length [cm].

The resulting Fulton’s condition factor (K) values were used to evaluate the physiological status of the individuals across different months and sexes. Descriptive statistics and frequency distributions were generated to summarize the variations in K values within the population.

The relationship between wet and gutted weights was initially examined using linear regression analysis. Regression models were fitted to the data, and the residuals were evaluated for deviations from linearity and violations of the assumption of independence. Regression coefficients and coefficients of determination (R²) were calculated to quantify the relationship strength. Additionally, the mean ratios of gutted, viscera, and gonad weights were calculated for each wet weight class using paired observations.

Statistical analysis. The wet weight (WW) classes were defined by equal binning, with individuals <100 g classified as juveniles. The significance of the regression was assessed using the F-statistic, and the b value for each class was tested using the t-test to check whether it differed significantly from isometric growth (b = 3). All statistical analyses were conducted using IBM SPSS v27 software.

Descriptive statistics of the biometric parameters for Holothuria tubulosa individuals (n = 327) sampled from the northeastern coast of the Aegean Sea revealed considerable variability in body size and composition. Wet weight ranged from 27.25 g to 440.14 g, with a mean of 144.94 ± 60.99 g, indicating a wide distribution of biomass and supporting the presence of multiple weight classes within the population (Fig. 2). Total body length varied between 9.00 cm and 30.00 cm, averaging 18.55 ± 3.98 cm. Body wall weight, representing the marketable portion of the sea cucumber, showed a mean value of 69.31 ± 21.06 g, with a minimum of 15.00 g and a maximum of 150.38 g. Viscera weight, which includes internal organs such as intestines and respiratory trees, was recorded for 301 individuals and averaged 31.69 ± 15.64 g. Gonad weight, measured in 274 individuals, exhibited high variability, ranging from 0.01 g to 35.16 g, with a mean of 5.01 ± 6.79 g.

Figure 2. 

Wet weight-frequency histogram of Holothuria tubulosa at from Ayvalık, northeastern Aegean Sea, Türkiye (n = 327).

The nonlinear regression analysis of the length–weight relationship for the sampled H. tubulosa revealed negative allometric growth, as indicated by the estimated allometric exponent b = 1.458, which was significantly different from the isometric value of 3 (P < 0.001). Growth dynamics did not differ among the LWR models fitted for sex and month. The coefficient of determination (R² = 0.565) for the overall samples indicated a moderate correlation between weight gain and length, showing that a 56.5% increase in weight depends on the increase in total body length. Across sex categories, the lowest coefficient of determination was 0.464 in females and 0.531 in males, while the highest value was calculated for indeterminate individuals with no gonads. The monthly R² values ranged from 0.012 to 0.734 (Table 1). Graphs of the regression analysis of the length–weight relationship are presented in Fig. 3.

Figure 3. 

Length–weight relationship of the Holothuria tubulosa population modeled using the nonlinear equation W = a∙L^b, where W is the wet weight and L is the total length. (A) The overall regression curve indicates negative allometric growth with an estimated exponent b = 1.458. Separate models fitted for (B) males and (C) females revealed distinct growth patterns, with males exhibiting b = 1.208 and females b = 1.584, which were significantly different from the isometric value of 3 (P < 0.001).

The condition factor (K), an indicator of individual health and robustness, was evaluated across the entire sample (n = 327). The mean condition factor was 2.42 ± 1.04, with individual values ranging from 0.51 to 6.66. Across sex classes, females (n = 142) had a mean condition factor of 2.32 ± 0.91, males (n = 109) had 2.42 ± 1.06, and indeterminate individuals (n = 76) had 2.62 ± 1.21. There were no statistically significant differences among the sex classes (P > 0.05).

The results of ANOVA followed by Tukey’s HSD post hoc test revealed significant pairwise differences in K among several months (P < 0.05). February and October exhibited the highest mean K values (3.26 ± 1.17 and 3.31 ± 1.26, respectively), which were significantly greater than those observed in September, July, May, June, and August, all of which formed a homogeneous group with the lowest K values (ranging from 1.91 ± 1.09 to 2.28 ± 0.89). Intermediate K values were found in March, April, November, January, and December, which were not significantly different from either the low or high K groups, suggesting that they represent a transitional category. These findings indicate a clear seasonal pattern in the condition factor, with lower values during the middle months and peaks in late winter and early autumn (Fig. 4).

Figure 4. 

Monthly variation in the condition factor (K) of individuals of Holothuria tubulosa from Ayvalık, northeastern Aegean Sea, Türkiye. The boxplot displays the interquartile range (IQR) and median. Statistical differences among months were assessed using one-way ANOVA, followed by Tukey’s HSD post-hoc test. Superscript letters indicate significant pairwise differences at the 0.05 significance level.

The condition factor of H. tubulosa varied across different wet weight classes, showing a general decreasing trend with increasing biomass. The highest mean K values of 2.89 ± 1.06 and 2.78 ± 0.85 were observed in the smallest and heaviest weight classes, respectively. Intermediate weight classes (100–150 g and 150–200 g) showed moderate K values (2.44 ± 1.04 and 2.20 ± 1.08, respectively), while heavier weight classes (200–250 g and 250–300 g) exhibited lower mean K values overall (2.01 ± 0.63 and 2.07 ± 0.85, respectively).

The Tukey HSD post hoc test comparing K across different weight classes of H. tubulosa revealed significant pairwise differences between the smallest weight class (<100 g) and several other classes (P < 0.05). Specifically, individuals in the <100 g class had significantly higher K values than those in the 150–200 g (mean difference = 0.69, P = 0.001) and 200–250 g classes (mean difference = 0.88, P < 0.001). A significant difference was also found between the <100 g and 100–150 g classes (mean difference = 0.45, P < 0.05), with the smaller individuals again exhibiting higher K values. Similar results were evident for the >300 g class. There were significant differences between the >300 g group and the 100–150 g, 150–200 g, and 200–250 g classes (P < 0.05), but not with the <100 g group (P > 0.05). No significant differences were detected among the intermediate or heavier classes (100–150 g to 250–300 g), indicating relative homogeneity in K values across these groups. These results suggest that smaller individuals of H. tubulosa tend to have better overall condition (as indicated by higher K values) than their heavier counterparts (Fig. 5).

Figure 5. 

Boxplot illustrates the distribution of the condition factor (K) across wet weight size classes of Holothuria tubulosa from Ayvalık, northeastern Aegean Sea, Türkiye. Boxes represent the interquartile range, with horizontal lines indicating the median values. The whiskers and outliers visualize the spread and variability. Significant differences among size classes were determined using one-way ANOVA followed by Tukey’s HSD post-hoc test, with Superscript letters denoting statistically significant pairwise comparisons (P < 0.05).

The linear regression model describing the relationship between wet weight and gutted weight demonstrated a statistically significant positive association (P < 0.001), with the regression equation defined as BW = 32.23 + 0.26 × WW. The coefficient of determination (R² = 0.549) indicated that approximately 54.9% of the variation in gutted weight could be explained by wet weight. Examination of the residuals revealed a generally uniform distribution around zero, with some indication of increased variance at higher wet weights, suggesting mild heteroscedasticity (Fig. 6). Nonetheless, the overall residual pattern supported the suitability of the linear model for describing the relationship across the observed weight range.

Figure 6. 

Residual plot of the linear regression model examining the relationship between wet and gutted weights of Holothuria tubulosa from Ayvalık, northeastern Aegean Sea, Türkiye. Residuals were plotted against wet weight to assess model fit and potential heteroscedasticity. The horizontal red dashed line represents a zero-residual reference.

The results revealed a clear trend of decreasing gutted weight percentages with increasing total wet weight. In the smallest weight class (<100 g), gutted weight constituted 57.08% ± 8.30 of total wet weight, ranging from 37.42% to 88.97% (n = 64). Viscera weight in this class averaged 20.28% ± 5.72, ranging from 11.02% to 37.74% (n = 53). Gonad weight was relatively low, averaging 1.53% ± 3.15 (n = 33), while total water content was 21%. As weight class increased, gutted weight percentage declined progressively: 54.88% ± 9.02 in the 100–150 g class (n = 136), 47.07% ± 7.71 in the 150–200 g class (n = 74), and 39.80% ± 8.30 in the 200–250 g class (n = 35). The largest class (>300 g) exhibited the lowest gutted weight percentage at 28.42% ± 5.82 (n = 6). Viscera weight percentage showed a slight increase from 20.28% in the smallest class to a peak of 26.39% ± 8.76 in the 150–200 g class, followed by a gradual decline to 18.09% ± 5.46 in the >300 g class. Gonad weight percentage increased with size, peaking at 5.63% ± 3.35 in the largest class. Water content also showed an increasing trend with size, rising from 21% in the smallest class to 48% in the largest class. These results suggest a size-dependent shift in body composition, with larger individuals exhibiting proportionally lower gutted weight and higher gonad and total water contents (Table 2). Mean viscera weights began to increase in March across monthly classes, reaching their highest levels in April, May, and June. Subsequently, moderate weights were recorded between July and October (Fig. 7A). Similarly, mean gonad weights showed an increasing trend starting in March, coinciding with the onset of gonadal development, and peaked in June. The lowest mean gonad weights were observed between September and February (Fig. 7B).

Figure 7. 

Mean (A) viscera and (B) gonad weights of Holothuria tubulosa from Ayvalık, northeastern Aegean Sea, Türkiye in month classes.

Biometric data collected from Holothuria tubulosa individuals sampled along the northeastern coast of the Aegean Sea revealed substantial variability in body size and composition, indicative of a heterogeneous population structure. Numerous studies have highlighted the high morphological plasticity of sea cucumber species, largely attributed to the variable water content within their skeletal and respiratory systems (Bulteel et al. 1992; Kinch et al. 2008; Zang et al. 2012; Prescott et al. 2015; Tolon et al. 2017; Ramírez-González et al. 2020). This physiological variability often introduces inconsistencies in the data used to establish reliable length–weight relationships (González-Wangüemert et al. 2014; Yussuf and Yahya 2020; Pasquini et al. 2021; Hammond and Purcell 2024). In weight determinations, wet weight is often favored because of its practicality and rapid application in the field, as well as its ability to reflect the natural morphology of individuals more accurately. However, in studies requiring high precision, as emphasized by Dereli et al. (2016), gutted weight is the most reliable measurement, obtained by dissecting sea cucumbers and weighing the body wall. Nonetheless, this approach may introduce bias in total length measurements owing to contraction of the body wall during dissection. In the presently reported study, both wet and gutted weights were recorded; however, due to excessive contraction of specimens during dissection from mouth to anus, only the wet total length was used in subsequent calculations. The wide range in wet weight (27.25–440.14 g) and total length (9.00–30.00 cm), along with mean values of 144.94 ± 60.99 g and 18.55 ± 3.98 cm, respectively, suggests the presence of multiple size and age classes within the population. The average total body length varied between 9.00 and 30.00 cm, with an average of 18.55 ± 3.98 cm, which is consistent with other studies in the region, which reported TL values of 16.4 cm (González-Wangüemert et al. 2015), 13.26 cm (Aydın 2020), and 19.8 cm (Lök et al. 2022). Variability is commonly observed in natural populations of sea cucumbers and reflects differences in growth rates, reproductive status, and environmental conditions (Purcell et al. 2016; Aydın 2020). The body wall, representing the commercially valuable portion of sea cucumbers, exhibited an average weight of 69.31 ± 21.06 g in the presently reported study. This value aligns closely with the gutted weight of 66.51 g reported by Aydın (2020) for specimens collected from the same location. Notable regional differences were observed when comparing these results with data from other parts of the northeastern Atlantic and the Mediterranean. For instance, González-Wangüemert et al. (2016) documented a higher mean body wall weight of 83.8 g, whereas Dereli et al. (2016) reported a considerably lower average weight of 49.9 g. This disparity highlights the fishing pressure on natural stocks in the northeastern Aegean region, where harvesting activities have particularly targeted larger individuals. Similarly, Kazanidis et al. (2010) reported that H. tubulosa individuals collected from Greek coasts, where sea cucumber fishing is either undeveloped or strictly regulated, had a considerably higher mean gutted weight of 108.46 g compared to those from Turkish waters. This suggests that sea cucumber populations along the Turkish coastline may be approaching an overexploited status.

The length–weight relationship analysis of H. tubulosa revealed a negative allometric growth pattern, with an allometric exponent b = 1.458, significantly lower than the isometric value of 3. This finding is consistent with previous studies on sea cucumbers from the genus Holothuria, where the increase in length often exceeds the increase in weight (Ahmed et al. 2018; Veronika et al. 2018; Aydın 2020). Negative allometric growth is typically associated with thinner or lighter body walls in longer individuals, which may influence the commercial value of the species. Several studies have indicated negative allometric growth values of 0.81 (Kazanidis et al. 2010), 1.06–1.67 (Dereli et al. 2016), and 1.725 (Pasquini et al. 2021) for H. tubulosa in the Mediterranean Sea, consistent with the findings of this study. Separate models fitted for males and females revealed distinct growth patterns, with males exhibiting b = 1.208 and females b = 1.58.

The condition factor (K) analysis indicated moderate variability among individuals, potentially reflecting differences in nutritional status, reproductive stage, or environmental conditions. Seasonal and sex-based variations in K values suggest biological influences on the physiological condition of the population. Notably, smaller or younger individuals tended to exhibit higher condition factors, indicating better health and robustness. This trend may be attributed to differences in metabolic demands and energy allocation strategies across growth stages, with juveniles possibly investing more energy in somatic growth and maintenance than in reproduction. Regression analysis between wet and gutted weights revealed a strong linear relationship, reinforcing the applicability of morphometric conversions for biomass estimation in this study. Interestingly, while the gutted weight ratio, representing the marketable portion, showed a relative decline with increasing total wet weight, the weight ratios of the viscera and gonads exhibited an upward trend. This pattern suggests a shift in internal body composition associated with physiological and reproductive processes. The viscera, comprising the intestines, gonads, and respiratory trees, represent a substantial and variable component of the total body mass, influenced by seasonality, sex, and reproductive status. These internal changes are indicative of reproductive investment and metabolic activity, and they complement condition indices such as Fulton’s K.

Seasonal variation in gonad development was evident in the sampled population, with increased gonad weights observed during the summer months (Fig. 7B), consistent with previous reports on the breeding season of this species (Kazanidis et al. 2010, 2014; Dereli et al. 2016). Mean gonad weights of H. tubulosa were not assessed across the entire annual population because of seasonal reproduction characteristics. However, they were included in the proportional morphological analyses to evaluate their contribution relative to the viscera.

The proportional increase in gonad weight during reproductive development, along with the intestines, serves as a reliable indicator of reproductive output and regional trophic conditions. The observed increase in viscera weight beginning in March may be attributed to increasing ambient seawater temperatures, which likely stimulate feeding activity and initiate gonad development, as supported by previous studies (Yang et al. 2005; Tahri et al. 2019; Pasquini et al. 2022; Engin et al. 2024). Consequently, the proportion of body wall weight relative to total wet weight is expected to decline during the reproductive period because of the internal growth of non-commercial tissues.

Overall, the biometric and physiological data presented in this study provide valuable insights into the growth dynamics and health status of H. tubulosa populations in the northeastern Aegean Sea. Integrating seasonal, sex-based, and size-related variability into length–weight and condition factor models enhances the precision of stock assessments and supports the development of adaptive management strategies. These findings contribute to a broader understanding of sea cucumber biology and underscore the importance of local data for sustainable fisheries management.

This research was funded by the Scientific and Technological Research Council of Türkiye (TÜBİTAK) under Grant No. 118O408. The author expresses sincere gratitude to TÜBİTAK for its financial support, which made this research possible. Special thanks are also extended to colleagues and collaborators whose insights and encouragement contributed significantly to this study.