Estimation of Population Parameters of Glossogobius giuris in the Rabnabad Channel, Southern Bangladesh: Implications for Sustainable Management

Abstract

The tank goby Glossogobius giuris is a commercially important dominant fish species in the Rabnabad Channel in southern Bangladesh. However, information on the population parameters of this species is not available to support its sustainable management. Therefore, the present study was undertaken to estimate the population parameters to understand both the current status and yield, and to suggest sustainable management measures for this species, using monthly samples collected from September 2021 to August 2022. Our results showed that the size of this species at first sexual maturity was 8.5 cm in total length (TL). The gonadosomatic index indicated a prolonged spawning season, with three peaks in January–February (minor peak), April–May (minor peak), and August–November (major peak). Recruitment occurred at ~8.2 cm TL for an extended period of the year with three pulses in March (major pulse), May–June (minor pulse), and December (minor pulse). The von Bertalanffy growth parameters were TL_∞ = 25.0 cm and K = 1.10 year⁻¹. The growth performance index and longevity were 2.84 and 2.7 years, respectively. The estimated fishing, natural, and total mortalities were 0.42, 2.00, and 2.42 year⁻¹, respectively. Therefore, the exploitation rate was 0.17, and the maximum sustainable yield was 0.37, indicating that the stock of G. giuris was not subjected to overexploitation. Hence, management intervention is not needed at this moment. Rather, a substantial amount of fishing pressure could be increased to obtain the maximum benefit.

1. Introduction

The fishing industry supports livelihoods, food security, and human health throughout the world. However, the sustainability of the fishing industry has become a global concern because of the rapid increase in fishing pressure and arbitrary exploitation. As a result, stocks are declining to levels that threaten ecosystems and societies [1]. This situation is worse, especially in developing countries where there is a lack of proper management tools and political will, and where the persistent illegal fishing of juvenile and brood fishes threatens the sustainability of fisheries [2]. Furthermore, the commoditization of fish and the pursuit of economic growth via industrialization and market expansion in developing countries drive overexploitation and may be more important than the weak institutions mentioned above [3,4]. Considering the worst-case scenario of the stocks, it is crucial to formulate effective management tools, focusing on all concerned stakeholders’ participation to promote the conservation and sustainable perpetuity of these fisheries [2,5]. An effective management plan incorporates various management tools, such as effort control, mesh size restriction for gears, allowable catch size, and seasonal closure of the fishery [6,7]. Before setting up these management tools, it is important to address the stock status first, especially the current level of exploitation and stock spawning biomass status.

Glossogobius is one of the largest genera of the Gobiidae family comprising at least 32 described species and several undescribed species [8]. The species of this genus are widely distributed in the tropical Indo-Pacific region, extending from East Africa to the Caroline Islands. Most species are benthic, riverine predators that are restricted to freshwater as adults, although a few species are found in estuaries [8]. The latter assemblage of species is suspected to be amphidromous with a marine larval stage, which accounts for their wide distribution [9]. G. giuris is widely distributed in fresh and brackish waters in the Indo-Pacific regions [10]. Fish have a special consumer preference in the diet of South Asian people because of their unique taste, low fat, and high protein content [11].

In Bangladesh, this species is primarily found in all kinds of fresh and estuarine waters and is of great importance due to consumer demand and high market value [12]. Out of the 18 gobiid species so far recorded in Bangladesh [13], the majority do not constitute an important fishery because of their small size and low consumer demand and market value. But G. giuris, which grows about a foot in length and has consumer demand and high market value, remarkably forms a fishery of some magnitude in the southern part of Bangladesh [14]. Therefore, this fish is caught in large quantities in both subsistence and artisanal fisheries in this area, including Rabnabad Channel. The latest study identified a total of 54 species from this channel [15], where G. giuris was in the fifth position based on abundance.

Numerous studies have been undertaken on this species from its major distribution areas, such as ecology and ecosystems [16,17,18,19,20,21], food and feeding habits [22,23,24,25,26,27,28,29], size relationships and growth [30,31], reproductive traits [14,16,22,24,28,32,33,34,35,36], and stock assessment [37]. However, there is no research on population parameters (e.g., reproduction, recruitment, growth, and mortality) of this species in Bangladeshi waters, which would help to estimate the maximum sustainable yield (MSY) and formulate fisheries management approaches for the resources [38]. Among the population parameters, growth and mortality help to determine the MSY, whereas information about various aspects of reproduction and recruitment helps in formulating management measures. Therefore, the aim of this study was to estimate the population parameters of G. giuris, which may provide a basis for the sustainable utilization and management of this species in the Rabnabad Channel in southern Bangladesh.

2. Materials and Methods

2.1. Study Site and Sampling

The present study was carried out in the Rabnabad Channel (21°52′ N, 90°16′ E), a vital navigational channel in the southern part of Bangladesh (Figure 1). The long channel stretching about 4 km was formed by merging the downstream of two rivers, Galachipa and Tentulia, and flowing into the Bay of Bengal. The channel is rich in aquatic resources, which play an important role in supporting the livelihoods of thousands of fishers [15]. A number of commercially important fish species, including G. giuris, are fished by small-scale artisanal fishers throughout the year. Monthly samples of the population were collected from September 2021 to August 2022 from the fishermen’s catch, who used a set bagnet to catch fish. After collection, the samples were immediately preserved in ice and then fixed with 10% formalin upon arrival at the laboratory. The collection details of G. giuris are shown in

Table 1. Collection record of G. giuris from the Rabnabad Channel, southern Bangladesh.

Sampling Month	Total Fish	No. of Males	Size Range		No. of Females	Size Range
			TL (cm)	BW (g)		TL (cm)	BW (g)
September 2021	126	39	8.4–12.5	8.1–46.4	87	10.0–14.4	26.4–565.8
October	146	63	7.0–19.5	8.3–141.6	83	8.0–15.0	12.7–57.1
November	130	82	6.5–14.0	8.7–63.9	48	9.0–13.5	18.2–57.9
December	119	56	4.5–11.0	3.1–27.9	63	6.4–13.3	8.2–60.4
January 2022	133	60	6.5–8.5	7.3–16.7	73	6.2–9.0	7.4–18.9
February	140	81	4.9–9.4	4.1–24.1	59	6.6–9.8	9.5–22.8
March	125	70	4.0–19.0	1.9–118.5	55	6.5–12.0	5.5–25.5
April	127	85	5.5–15.0	7.0–116.2	42	6.5–16.5	8.9–128.4
May	137	78	4.5–16.0	3.4–103.7	59	6.0–13.0	6.9–51.8
June	136	87	5.0–19.5	3.8–182.9	49	6.0–14.0	7.1–68.5
July	146	95	7.5–12.0	10.9–41.0	51	8.0–15.5	12.6–85.5
August	137	93	5.5–13.2	4.6–49.2	44	6.5–12.0	10.2–39.0

.

2.2. Fish Measurement

The fixed specimens were measured for (i) total length (TL) using a measuring scale to the nearest 0.1 cm, and (ii) body weight (BW) using a digital balance (AND, FSH, Republic of Korea) to 0.01 g accuracy. Specimens were sexed by abdominal incision and visual inspection of the gonad. To estimate the parameter reproduction (e.g., length at first sexual maturity and spawning season), only female individuals were used, whereas other parameters were estimated using combined sexes.

2.3. Sexual Maturity and Spawning Season

Whole ovaries were removed from each female, and fat, connective tissue, and blood vessels were carefully removed before weighing to the nearest 0.001 g (OW). The gonadosomatic index (GSI) was calculated as follows:

GSI (%) = 100 × OW/BW

(1)

This index assumes that the gonad increases in size with development. Size at first sexual maturity was determined by the relationship between TL and GSI of all females, and the smallest female with an advanced development ovary was considered the minimum size of maturity for the population. The spawning season was estimated based on the monthly variation in GSI, and the results were correlated with monthly air temperature, rainfall, and photoperiod data obtained from World Weather Online (www.worldweatheronline.com/kuakata-weather-averages/bd.aspx, accessed on 15 February 2022) using the Spearman rank correlation test.

2.4. Recruitment Pattern

Length–frequency distribution using TL data pooled from all monthly samples of 1 cm class intervals was constructed. A series of component normal distributions were fitted to the frequency distribution using the program FiSAT II [39] based on the method by Bhattacharya [40]. Each identified normal distribution was assumed to present a distinct age group in the population. The outputs from this analysis include the mean TL and standard deviation explained by each component’s normal distribution. The mean TL of the smallest size group was considered the length at recruitment of G. giuris in the Rabnabad Channel, and individuals with mean + SD TL or smaller sizes were considered recruits in the stock [41]. The yearly recruitment pattern was determined by the occurrence of the monthly percentage of recruits.

2.5. Growth Analysis

Monthly length–frequency data using 1 cm class intervals of TL for combined sexes were inputted into the FiSAT II software [39] to estimate the growth parameters using the following von Bertalanffy [42] equation:

L_t = L_∞ [1 − exp{−K(t − t₀)}]

(2)

where L_t is the TL (cm) at age t (month), L_∞ is the asymptotic TL (cm), K is the growth coefficient (year⁻¹), and t₀ is the hypothetical age when TL would be zero. First, we estimated L_∞ and Z/K from the Powell–Wetherall plot [43,44] given by a linear regression equation:

L_m–L′ = a + bL′

(3)

where L_m = the mean length of all fish, Lʹ = the cut-off length, a = the intercept, and b = the slope. L_∞ and Z/K were computed using the above equation as follows:

L_∞ = a/b

(4)

Z/K = −(1 + b)/b

(5)

This initial L_∞ was used as the seed value to fit the von Bertalanffy growth function (VBGF) to the length–frequency data using the ELEFAN I procedure. The best growth curve was identified based on the index of goodness-of-fit (Rn), along with the final L_∞ and K values.

2.6. Growth Performance and Longevity

The growth performance index (Ø’) [45] was calculated using the estimated values of L_∞ and K of the VBGF as follows:

Ø’ = log₁₀ K + 2log₁₀L_∞

(6)

Longevity (t_max) was estimated as t_max ≈ 3/K [46,47].

2.7. Estimation of Mortality and Exploitation Rate

Total mortality (Z) was estimated using the length-converted catch curve method [48] as follows:

ln(N_t/Δ_t) = a + bt

(7)

where N = the number of individuals of relative age (t) and Δ_t = time needed for the fish to pass through a length class. The slope b of the curve, with its sign changed, provides an estimate of Z [41]. Natural mortality (M) was estimated using the following formula [47]:

log₁₀M = −0.006 − 0.279 log₁₀L_∞ + 0.654 log₁₀K +0.463 log₁₀T

(8)

where T = the average annual water temperature (°C) of the habitat. Fishing mortality (F) and exploitation rate (E) were calculated from F = Z − M and E = F/Z, respectively.

2.8. Maximum Sustainable Yield

We calculated the relative yield-per-recruit (Y′/R) using the length-based Beverton and Holt [49] methods. The relative biomass per recruit (B′/R) was obtained from B′/R = (Y′/R)/F. Then, the maximum sustainable yield (E_max), the exploitation rate with the minimal increase of 10% of Y′/R (E_0.1), and the exploitation rate with the reduction of stock to 50% (E_0.5) were estimated using knife-edge selection [49].

3. Results

3.1. Sexual Maturity

Figure 2 illustrates the relationship between TL and GSI of female G. giuris. The lowest and highest GSIs were recorded during the study period as 0.03 and 16.97, respectively. It was found that the GSI value was relatively lower up to a female size of >8.5 cm. However, beyond this size, the GSI value rose sharply. Therefore, the size at first sexual maturity of G. giuris was considered to be 8.5 cm TL at the study site.

3.2. Spawning Season

Figure 3 shows the monthly changes in the mean GSI of female G. giuris in relation to environmental factors. The mean GSI increased sharply in January, started to decrease afterward and declined significantly in March. The GSI again started to increase in April, remained high till May, and then began to decrease sharply until July. Thereafter, the GSI started to increase from August, remained high from September to October, and then gradually decreased in the subsequent months. Therefore, the GSI of G. giuris indicated a prolonged spawning season with three peaks in January–February (minor peak), April–May (minor peak), and August–November (major peak). There were no correlations between spawning season and monthly air temperature (r_s = 0.214, p = 0.502), rainfall (r_s = −0.399, p = 0.199), and photoperiod (r_s = −0.392, p = 0.210).

3.3. Recruitment Pattern

We detected four distinct normal distributions using the Bhattacharya [40] method, each of which represents a size group with their mean TL (Figure 4A). The calculated mean length of the smallest size group 8.2 cm TL was considered as the length at recruitment of G. giuris in the Rabnabad Channel where fishing occurred. The yearly recruitment indicated an extended pattern with three pulses in March (major pulse), May–June (minor pulse), and December (minor pulse) (Figure 4B).

3.4. Growth Analysis

The analysis of the length–frequency data by the Powell–Wetherall plot provided initial estimated values of TL_∞ = 24.6 cm and Z/K = 3.71 (Figure 5). Using this initial TL_∞ as the seed value, the ELEFAN I analysis yielded an optimized von Bertalanffy growth curve with the following parameters: TL_∞ = 25.0 cm and K = 1.10 year⁻¹. The third parameter of the von Bertalanffy growth function t₀ was assumed to be zero [50]. The graphical outputs of the growth curve are presented in Figure 6. The estimated growth performance index and the longevity of G. giuris were 2.84 and 2.7 years, respectively.

3.5. Mortality and Exploitation Rate

We obtained a Z value of 2.24 year⁻¹ from the length-converted catch curve, and the graphical outputs are presented in Figure 7. The average annual water temperature of the Rabnabad Channel is 28 °C. The calculated value of M and F were 2.00 and 0.42 year⁻¹, respectively. Therefore, the calculated value of E for G. giuris was 0.17 in the Rabnabad Channel.

3.6. Maximum Sustainable Yield

The analyses of relative yield per recruit and relative biomass per recruit showed the maximum sustainable yield E_max = 0.37, the optimum yield E_0.1 = 0.31, and the yield at the stock reduction of 50% E_0.5 = 0.24 (Figure 8).

4. Discussion

The purpose of fisheries management is to ensure that catches from a fish stock are ecologically sustainable in the long term and that benefits to fishers and communities are maximized. To formulate explicit management measures, a fish stock needs information on its parameters, namely, reproduction (e.g., sexual maturity and spawning season), recruitment, growth, and mortality. The first two parameters were used to formulate management measures, while the latter two parameters helped in the stock assessment. Therefore, the present study attempted to provide necessary information for the fisheries management of G. giuris from Bangladeshi waters.

Diversified methods are used for the traditional data-rich and data-poor stock assessments [51]. However, the data required for traditional stock assessments are rarely available for most of the exploited fisheries, especially in developing countries [52,53]. Currently, two types of methods are commonly used in data-poor fisheries: (i) catch-based methods and (ii) length-based methods [54]. The catch-based methods require catch time series and supplementary data, e.g., intrinsic rate of increase, natural mortality, and age at maturity, whereas the length-based methods require length–frequency data. In the present study, we used electronic length frequency analysis (ELEFAN), which is one of the widely used length-based methods.

Determination of sexual maturity (L_m) is imperative for the assessment of the minimum permissible capture size of exploited stocks [55,56]. This information helps in the mesh size selection of the gear to restrict the catching of immature individuals, thus giving them an opportunity to spawn at least once in their life cycle. In our study, the size at first sexual maturity of G. giuris was found to be 8.5 cm TL based on the relationship between TL and GSI. The size at sexual maturity of some gobiid species is summarized in

Table 2. Size at sexual maturity in some gobiid species.

Species	Lm/TL	Lm (cm)	Locality	Reference
Glossogobius giuris	0.58	8.5	Gajner Beel, Bangladesh	[57]
	0.25–0.32	4.8–6.1	Mekong Delta, Vietnam	[36]
	0.38	9.5	Manchar Lake, Pakistan	[35]
	0.35	8.5	Rabnabad Channel, Bangladesh	Present study
Periophthalmus barbarus	0.79	10.2	Imo River, Nigeria	[59]
Pseudapocryptes elongatus	0.69	15.4	Mekong Delta, Vietnam	[60]
Boleophthalmus boddarti	0.68	11.5	Mekong Delta, Vietnam	[61]
Parapocryptes serperaster	0.69	15.8	Mekong Delta, Vietnam	[62]
Stigmatogobius pleurostigma	0.73	4.1	Mekong Delta, Vietnam	[63]
Trypauchen vagina	0.84	16.6	Mekong Delta, Vietnam	[64]
Periophthalmodon septemradiatus	0.57–0.74	6.05–6.78	Mekong Delta, Vietnam	[65]
Butis koilomatodon	0.43–0.67	4.8–6.7	Mekong Delta, Vietnam	[66]
Glossogobius sparsipapillus	0.42–0.68	6.1–8.9	Mekong Delta, Vietnam	[67]

. We used the L_m/TL ratio to compare L_m between or within species as the total length changes with species. Our result is similar to that of a recent study by Hasan et al. [57] from Gajner Beel, Bangladesh. However, the L_m/TL ratio in the present study was lower than that in the previous study [57], indicating that this species in Rabnabad Channel matured earlier than in Gajner Beel. It can be noted that the Gajner Beel is a freshwater habitat, and the Rabnabad Channel is a coastal habitat. Therefore, the early maturation of G. giuris in the present study compared to the previous study in Gajner Beel is related to salinity as also reported by Dinh et al. [36]. The L_m of G. giuris may vary with location due to the divergent climatic and trophic parameters [58], as this value in the present study was lower than the study in Manchar Lake, Pakistan [35] and higher than the study in Mekong Delta, Vietnam [36]. The L_m/TL ratio indicates that G. giuris matured earlier than other gobies [59,60,61,62,63,64,65,66] and even earlier than its congener, G. sparsipapillus [67].

The GSI showed that G. giuris could release eggs throughout the year with three peaks: two minor peaks in January–February and April–May, and a major peak occurring in August–November. The most vital environmental factors directly or indirectly influencing the reproduction of aquatic animals are temperature, rainfall, and photoperiod [68,69,70,71,72,73]. The seasonality of these factors also influences biological cycles by ensuring larval occurrence during the periods of available food [73,74]. In our study, no significant distinct pattern of temperature, rainfall, or photoperiod was found to explain the seasonality of spawning. But if we could collect samples from at least two different sites with different environmental parameter values, we may detect significant correlations between the spawning season and environmental parameters. In fact, this was not possible in this case, as the present study was based on fishery-dependent data. However, relatively high temperatures (23–33 °C) throughout the year at our study site may cause year-round spawning of this species, as reported by several studies [38,71,72,73,74,75,76,77,78,79,80]. The spawning seasons of some gobiid species are summarized in

Table 3. Spawning season in some gobiid species.

Species	Spawning Season	Locality	Reference
Glossogobius giuris	December	Payra River, Bangladesh	[34]
	June–October	Mithamoin Haor, Bangladesh	[33]
	April–June	Manchar Lake, Pakistan	[35]
	April and September	Mekong Delta, Vietnam	[36]
	August–November	Rabnabad Channel, Bangladesh	Present study
Pseudapocryptes elongatus	July–October	Mekong Delta, Vietnam	[60]
Boleophthalmus boddarti	August–October	Mekong Delta, Vietnam	[61]
Parapocryptes serperaster	September	Mekong Delta, Vietnam	[62]
Stigmatogobius pleurostigma	March–November	Mekong Delta, Vietnam	[63]
Trypauchen vagina	June–August	Mekong Delta, Vietnam	[64]
Periophthalmodon septemradiatus	Year-round	Mekong Delta, Vietnam	[65]
Butis koilomatodon	April	Mekong Delta, Vietnam	[66]
Glossogobius sparsipapillus	July–November	Mekong Delta, Vietnam	[67]

. Dinh et al. [36] reported that G. giuris spawn throughout the year, with a peak in the late dry season for those inhabiting freshwater and in the wet season for those inhabiting brackish waters, thus salinity has a significant impact on the spawning season of this species. However, this statement does not concur with other studies on freshwater [33,34,35]. The main spawning season of G. giuris varied with location, but it can be concluded that this goby released eggs mainly from the late dry season (April) to the wet season (May–December). Comparisons among the gobies suggested that peak spawning was species-specific and varied with location.

The recruitment pattern of G. giuris showed an extended form with three pulses (a major pulse in March and two minor pulses in May–June and December) associated with three spawning peaks. Dinh et al. [37] also reported a prolonged recruitment pattern of this species with two peaks: a minor peak in March–April and a mean peak occurring in September. It seems that the variations in the recruitment time of this goby could be related to the variations in spawning time and location.

The estimated TL_∞ and K values in the present study were compared with the available studies on this and other gobiid species (

Table 4. Population parameters of some gobiid species.

Species	L∞	K	Ø′	tmax	Z	F	M	E	Locality	Reference
Glossogobius giuris	19.6	1.36	–	5.00	–	–	–	–	Titas River, Bangladesh	[31]
	20.5	0.56	2.37	5.36	3.17	1.77	1.40	0.56	Mekong Delta, Vietnam	[37]
	25.0	1.10	2.84	2.70	2.42	0.42	2.00	0.17	Rabnabad Channel, Bangladesh	Present study
Periophthalmus barbarus	21.6	0.55	2.41	5.45	4.21	2.86	1.35	0.68	Imo River, Nigeria	[59]
Boleophthalmus boddarti	16.8	0.79	2.35	3.55	2.13	0.30	1.83	0.14	Mekong Delta	[81]
Stigmatogobius pleurostigma	8.60	0.83	1.79	3.61	3.48	1.17	2.31	0.34	Mekong Delta	[82]
Butis koilomatodon	10.0	0.94	1.79	3.61	3.48	1.17	2.31	0.34	Mekong Delta	[83]
Glossogobius sparsipapillus	17.8	0.62	2.19	4.84	2.17	0.68	1.49	0.31	Mekong Delta	[84]
	15.9	0.83	2.32	3.61	3.46	1.78	1.68	0.51	Mekong Delta	[84]
Periophthalmodon schlosseri	29.0	1.40	3.10	2.14	–	–	–	–	Malaysia	[85]
Glossogobius aureus	28.0	0.72	2.75	4.16	4.25	2.73	1.52	0.64	Mekong Delta	[86]
Pseudapocryptes elongatus	26.0	0.65	2.64	4.35	2.91	1.47	1.44	0.51	Mekong Delta	[87]
Parapocryptes serperaster	25.2	0.74	2.67	4.05	3.07	1.57	1.51	0.49	Mekong Delta	[88]
Trypauchen vagina	24.2	0.56	2.50	5.56	2.73	1.29	1.44	0.53	Mekong Delta	[89]
Butis butis	24.0	0.61	2.55	4.92	3.4	1.98	1.42	0.58	Mekong Delta	[90]

). In comparison with other studies of G. giuris, our TL_∞ exceeded the value reported by Ahmed and Latifa [31] in Titas River, Bangladesh, and Dinh et al. [37] in Mekong Delta, Vietnam. In contrast to other gobiids, our estimated L_∞ value was higher than that of some species [59,81,82,83,84], lower than that of some species [85,86], and comparable with that of some species [87,88,89,90]. These differences in asymptotic length were species-specific and varied with the differences in environmental factors across habitats. The K-value in our study was lower than the value reported by Ahmed and Latifa [31] and higher than the value reported by Dinh et al. [37]. However, compared to other gobiids, our estimated K-value was in the upper range, except for Periophthalmodon schlosseri [85] (Table 4). Generally, L_∞ and K are intrinsically negatively correlated [45]. Therefore, it is recommended to use the growth performance index (Ø′) to compare growth between species rather than the comparison of L_∞ and K. The growth performance index helps in better comparisons of growth between species and/or habitats [91]. Our estimated Ø′ was higher than that of other gobiid species, except for Periophthalmodon schlosseri [85]. The difference in Ø′ could be due to the variations in K and L_∞ among these gobiids. The estimated longevity (t_max) of this fish was lower than that reported in previous studies [31,37], even though it was lower than that of the other gobiid species, except for Periophthalmodon schlosseri [85]. The t_max of fish can be influenced by several intrinsic and extrinsic factors.

To develop a sustainable management approach for an exploited fishery, it is imperative to know the current and desired levels of fishing mortality. Although many other matrices may regulate the fishery’s status for operational purposes, these two indices have been widely used to formulate management actions for obtaining the maximum sustainable yield by keeping the fishing pressure at a sustainable level to maintain the productivity of the stock [92]. In our study, fishing mortality (0.42 year⁻¹) was significantly lower than the natural mortality (2.00 year⁻¹), indicating that the survival rate depends more on the likely impact of environmental factors. Similar environmental impacts were also observed in the stock of some goby species living in the Mekong Delta, Vietnam (MDV), such as Boleophthalmus boddarti [81], Stigmatogobius pleurostigma [82], Butis koilomatodon [83], Glossogobius sparsipapillus [84], and Trypauchen vagina [89]. Our computed exploitation rate (E = 0.17) was much lower than our predicted maximum sustainable yield (E_max = 0.37), indicating that the stock of G. giuris was not subjected to overexploitation. Likewise, the stock of other goby species reported from Mekong Delta, such as Boleophthalmus boddarti [81], Stigmatogobius pleurostigma [82], Butis koilomatodon [83], Glossogobius sparsipapillus [84], Pseudapocryptes elongatus [87], and Parapocryptes serperaster [88], were not subjected to overexploitation. On the contrary, the only study [37] on this species and the studies on Glossogobius aureus [86], Trypauchen vagina [89], and Butis butis [90] stocks from Mekong Delta and Periophthalmus barbarus [59] stock from Imo River, Nigeria, were overexploited. The divergence of the fishing status of these gobiids could be related to the variations in fishing preferences.

5. Conclusions

This was a complete and rigorous study of G. giuris from Bangladeshi waters. The main purpose of this study was to assess the population parameters of G. giuris and to show how this information can support fishery managers in designing their management plans and managing the fishery. The results of the study indicate that G. giuris become mature at ~8.5 cm in total length and spawn throughout the year with three peaks with a subsequent occurrence of recruitment in three pulses. Their lifespan is ~2.7 years. The species is underexploited, and the situation does not require management intervention at present. Rather, a substantial amount (20%) of fishing pressure could be increased to obtain the maximum benefit from the fishery.