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Stock Assessment of Two Parrotfish, Hipposcarus harid and Scarus ferrugineus in Jeddah, Saudi Arabia

PJZ_52_5_1709-1722

 

 

Stock Assessment of Two Parrotfish, Hipposcarus harid and Scarus ferrugineus in Jeddah, Saudi Arabia

Ahmad Osman Mal1 and Mohamed Hosny Gabr1,2,*

1Marine Biology Department, Faculty of Marine Science, King Abdulaziz University, P.O. Box 80207, Jeddah 21589 Saudi Arabia

2National Institute of Oceanography and Fisheries, Suez, Egypt

ABSTRACT

The current stock status of two Parrotfish, Hipposcarus harid and Scarus ferrugineus in Jeddah was assessed. Scales were used for age determination and back calculations of length-at-ages. The growth parameters were estimated to be: the asymptotic length L = 54.044 cm, the growth coefficient K = 0.168 year-1, and age at zero length to = -0.707 year for H. harid and L = 51.238 cm, K = 0.170 year-1, and to = -0.889 year for S. ferrugineus. The total ‘Z’, natural ‘M’, fishing ‘F’ mortality coefficients and current exploitation ‘Ecur’ were 0.96, 0.294, 0.670 year-1, and 0.69 year-1, respectively for H. harid and 0.77, 0.299, 0.471 year-1, and 0.61 year-1 respectively for S. ferrugineus. The maximum yield per recruit at the current fishing mortality was 108.1 g for H. harid and 137.6 g for S. ferrugineus. The biological reference points: Fmax = 0.401 year-1 and F0.1 = 0.245 year-1 for H. harid and Fmax = 0.434 year-1 and F0.1 = 0.307 year-1 for S. ferrugineus were lower than the current fishing mortality, reflecting an over-exploitation for both species. The current fishing mortality is recommended to be decreased to the target reference point F0.1 = 0.453 year-1 for H. harid and F0.1 = 0.466 year-1 for S. ferrugineus after increasing the age at first capture to be 3 years for both species.


Article Information

Received 29 July 2017

Revised 12 May 2018

Accepted 06 August 2019

Available online 08 May 2020

Authors’ Contribution

AOM and MHG initiated and designed the study and collected the samples. MHG conducted the experiments and wrote the article. AOM reviewed the article.

Key words

Hipposcarus harid, Scarus ferrugineus, Age determination, Growth parameters, Mortality coefficients, Maximum yield per recruit.

DOI: https://dx.doi.org/10.17582/journal.pjz/20170729030714

* Corresponding author: mhosnyg@yahoo.com

0030-9923/2020/0005-1709 $ 9.00/0

Copyright 2020 Zoological Society of Pakistan



Introduction

Hipposcarus harid (Forsskål, 1775) and Scarus ferrugineus (Forsskål, 1775) are two species of Parrotfishes. Ongoing phylogenetic and evolutionary research on parrotfishes indicate that they are either a separate family (Scaridae) under the suborder Labroidei (Bellwood, 1994; Randal, 2007) or a subfamily (Scarinae) under the family Labridae (Westneat and Alfaro, 2005). Parrotfishes are a dominant group of herbivorous species that play an important role in bioerosion and hence affect the benthic communities’ structure on coral reefs (Williams and Hatcher 1983; Russ, 1984; Choat and Bellwood, 1991; Alwany et al., 2009).

Parrotfishes are also important fishery resources in coral reef small-scale artisanal fisheries and caught mainly in gillnet and pot fishing gears. As indicated by Lokrantz et al. (2008), the decline in the biomass and abundance of parrotfishes due to overfishing may have severe negative impacts on the coral reefs dynamics and regeneration. In Saudi Arabia, parrotfishes represent an important part of the catch of coral reef fisheries which are abundant along the Red Sea coast. The average annual catch of parrotfishes from the Red Sea coast of Saudi Arabia during the period from 2008–2016 was 364 tones (FAO, 2018). The two parrotfish species Hipposcarus harid and Scarus ferrugineus are a major component of parrotfish catch from Jeddah fisheries.

The growth rates and longevity in some scarid species have been reported in previous studies based on alternating light and dark bands in hard structures (Warner and Downs, 1977; Russ and St. John, 1988; Clifton, 1995; van Rooij et al., 1995). However, there are only a few studies on the age and growth of the two species in the Red Sea (Ali et al., 2011; Mehanna et al., 2014). There are no previous studies available on the stock assessment for these two parrotfish species in Jeddah fisheries.

The aim of the present study was to estimate age and growth using scales as hard structures, mortalities, length and age at first capture, yield and biomass per recruit of the two parrotfish species in Jeddah fisheries.

 

Materials and methods

Representative samples of the two parrotfish species Hipposcarus harid and Scarus ferrugineus were collected monthly from the daily fish auction held at the main fish landing site of Jeddah (main fish market) during the period from March 2013 to January 2014. Samples were collected randomly from the landed catch which is harvested using the different fishing gears in Jeddah (Fig. 1), to minimize biases in the sample size distributions produced by differences in gear selectivity (Goodyear, 1995).


 

For each specimen, the total fish length (L) was measured to the nearest 0.1 cm and total body weight (W) was recorded to the nearest 0.1 g. The power equation: W = a L b was used to describe the length -weight relationship, where a is the intercept and b is the slope estimated from the linear regression analysis of the following linearized form of the power equation:

ln W = ln a + b ln L

For age determination, scales from behind the left pectoral fin were collected, cleaned in water then dried and mounted between two microscopic glass slides. The mounted scales were examined under a stereo-zoom microscope (MEIJI) using a digital video camera connected to a personal computer, where Micrometrics SE Premium software was used to capture and save pictures for scale measurements.

Two linear regression analyses were used to describe the relationship between the body length (L) and scale radius (S) for the two species. The first one is the regression of L on S based on the linear form: L = c + d S (c is the intercept, d is the slope), and the second is the regression of S on L based on the linear form: S = e + f L (e is the intercept, f is the slope). The Lengths corresponding to previous years of life were estimated (back-calculated) using three back-calculation methods (Francis, 1990; Pierce et al., 1996):

Fraser-Lee equation (Lee, 1920):

Li = c + (L - c) (Si / S)

Body proportional hypothesis (BPH):

Li = [(c + dSi) / (c + dS)] L

Scale proportional hypothesis (SPH):

Li = -(e/f) + [L + (e/f)] (Si / S)

Where, L is the observed (at capture) fish length, S is the scale radius (at capture), Li is the back-calculated length at the time of annulus i formation, Si is the radius of the annulus i. The mean calculated lengths at ages estimated by the three methods were compared using a one-way analysis of variance (ANOVA) test, applied using ‘Statistix 8.1’ software (Analytical Software, Tallahassee, USA).

The asymptotic length (L) and growth coefficient (K) are two parameters of the von Bertalanffy (1938) growth equation (VBGE): Lt = L [1- e-K (t- t0)]. Both parameters were estimated using the method of Ford (1933) and Walford (1946) fitted to the average back-calculated lengths-at-ages. The third growth parameter (t0) (supposed age at zero length) was estimated, based on the estimated values of L and K, by re-arranging the von Bertalanffy growth equation as described by Sparre and venema (1998): t0 = t + 1/K Loge (1Lt / L).

The performance index of growth in length ‘ Ǿ ‘ for both species was calculated by the formula suggested by Pauly and Munro (1984): Ǿ = Log K + 2 Log L. Because this index is based on length which is rarely lost by fish, it is considered the most precise and flexible index of growth performance which can be used to compare growth performances of wild and cultured fish stocks (Mathews and Samuel, 1990).

The linearized length-converted catch curve method of Pauly (1983), implemented in the FiSAT II software (Gayanilo et al., 2005), was used to estimate the instantaneous total mortality coefficient ‘Z’. The natural mortality coefficient ‘M’ was estimated by the equation proposed by Jensen (1996) and modified by Hamel (2015) as follows: M = 1.753 K, where K is the growth coefficient. The difference between the total mortality coefficient ‘Z’ and the natural mortality coefficient ‘M’ was accounted to the fishing mortality coefficient ‘F’. The exploitation ratio ‘E’ was expressed by the ratio between F and Z (E = F/Z).

Three types of fishing gears; gillnets, trammel nets and fish pots are working on all sizes in the fish populations of H. harid and S. ferrugineus in Jeddah fisheries. Based on Crone et al. (2013), since the selectivity of at least one gear type; fish pots, is considered to be sigmoidal (similar to the trawl codend selectivity) (Stewart and Ferrell, 2001; Boutson et al., 2009; Songrak et al., 2013), it is assumed, in the present study, that the gear selectivity for H. harid and S. ferrugineus is asymptotic (sigmoidal) and thus the length at first capture ‘Lc’ (the mean selection length at which 50% of the fish that entered the fishing gear are retained) for both species was estimated using the cumulative method described by Pauly (1984b) implemented in the FiSAT II software. The age ‘ tc ‘ corresponding to Lc was estimated by applying the inverse von bertalanffy equation as follows: tc = t0 - 1/K Loge (1Lc / L).

The model of Beverton and Holt (1957) was used to estimate the yield per recruit Y/R and biomass per recruit B/R of H. harid and S. ferrugineus in Jeddah fisheries as follows:

Where, Y/R is yield per recruit, F is the fishing mortality coefficient, M is the natural mortality coefficient, tc is the mean age at first capture, tr is the mean age at recruitment, W is the asymptotic weight, Z is total mortality coefficient, K is the growth coefficient and S equal to the equation: S = e – K ( tc – t0 ). The biomass per recruit (B/R) was estimated by dividing the Y/R by the fishing mortality F.

Two yield-based biological reference points were determined at two levels of length at first capture Lc: Fmax (the fishing mortality level that produces the maximum yield per recruit) and F0.1 (the fishing mortality level at which the slope or marginal increment of the yield per recruit is 10% of its value at the origin, where E=0). The current fishing mortality level Fcur was matched with these reference points. Fmax is referred to as the ‘Limit’ reference point, while F0.1 is considered as the target reference point (Gabriel and Mace, 1999; Cadima, 2003; Hoggarth et al., 2006).

 

Results

Fishing gears

Scarid species, as herbivores, are usually caught with gillnets, trammel nets and fish traps (pots). Table I shows the specifications of gillnets and trammel nets used to catch most of the parrotfishes landed in Jeddah fisheries. Two fishermen on a wooden boat (7–9 m length) provided with outboard engine of 25–40 HP usually use some units of gillnets to set in the lagoons and trammel nets to set on the reef flat (mainly during daytime) to target parrotfishes among other reef fishes in the coral reef fisheries of Jeddah.

Age determination and back-calculations

By counting the number of annuli that appeared on the scales, the age in years could be assigned to each specimen. Twelve age groups (0-11) could be determined for H. harid based on scales reading of 667 specimens ranging from 12.4 – 48.2 cm in total length, while fifteen age groups (0-14) were observed for S. ferrugineus based on 516 specimens ranging in total length from 13.2–48.5 cm.

The relationship between the fish length and scale radius was found to be linear, as shown in Figure 2, and the model of linear regression of L on S was the best fit for this relationship where the regression standardized residuals plotted against predicted lengths showed no clear pattern but random scattering around zero line (Fig. 2). For back-calculations, the fish length - scale radius relationship could be described by the following two linear equations for each species, based on the linear regression analysis for pooled data (sexes combined):

 

Table I.- Specifications of gillnets and trammel nets used in Jeddah coral reef fisheries.

Gear Item

Gillnets

Trammel nets

Unit length

35 - 40 m

45 - 50 m

Net depth

1.0 – 1.2 m

0.9 – 1 m

Number of Units per boat

4 - 6

6 - 8

Float line material and diameter

Polyethylene, 5 - 6 mm

Polyethylene, 4 - 5 mm

Lead line material and diameter

Polyethylene, 4 - 5 mm

Polyethylene, 4 - 5 mm

Number and material of floats per unit

45-50 Cork floats, 25 gf

60-70 Cork floats, 25 gf

Distance between two floats

85 - 90 cm

60 - 65 cm

Number of lead sinkers per unit

70 – 75 (30 g each)

110 – 120 (35 g each)

Distance between two sinkers

65 - 70 cm

35 - 40 cm

Inner panel mesh size and twine diameter

62 - 88 mm; 0.3-0.4 mm

50 - 62 mm; 0.3 mm

Outer panel mesh size and twine diameter

Not present

150 - 170 mm; 0.4 mm

Twine material

Polyamide, Mono- or Multi-filament

Polyamide, monofilament

 

Table II.- The mean back-calculated lengths at ages estimated by three methods: Fraser -Lee, Body proportional hypothesis (BPH) and Scale proportional hypothesis (SPH) for H. harid and S. ferrugineus collected from Jeddah fisheries.

Age

H. harid

S. ferrugineus

Fraser-Lee (pooled)

BPH

SPH (pooled)

Fraser-Lee (pooled)

BPH

SPH (pooled)

Females

Males

Pooled

Females

Males

Pooled

1

13.83

14.26

14.58

13.82

12.90

14.59

14.81

14.54

14.59

13.89

2

20.72

20.82

21.20

20.71

20.36

21.09

21.37

20.97

21.09

20.71

3

25.80

25.82

25.65

25.79

25.52

25.81

25.29

26.10

25.81

25.55

4

29.72

29.34

29.73

29.71

29.39

29.22

29.16

29.27

29.21

28.93

5

32.98

32.80

33.24

32.97

32.66

32.21

32.80

32.18

32.21

31.89

6

35.84

-

35.96

35.83

35.38

35.24

36.42

35.11

35.24

35.01

7

38.50

-

38.60

38.49

38.09

37.45

38.65

37.9

37.44

37.15

8

41.13

-

41.20

41.12

40.87

39.39

40.30

39.11

39.38

39.17

9

43.53

-

43.57

43.52

43.36

41.16

41.73

41.00

41.16

41.02

10

45.30

-

45.34

45.29

45.14

42.79

-

42.80

42.78

42.68

11

46.78

-

46.79

46.77

46.70

44.30

-

44.30

44.29

44.24

12

-

-

-

-

-

45.58

-

45.59

45.58

45.47

13

-

-

-

-

-

46.69

-

46.70

46.69

46.62

14

-

-

-

-

-

47.60

-

47.60

47.59

47.56


 

For H. harid (R2 = 0.917):

L = 2.901 S – 2.175 (regression of L on S)

S = 1.549 + 0.316 L (regression of S on L)

For S. ferrugineus (R2 = 0.930):

L = 2.564 S – 1.726 (regression of L on S)

S = 1.321 + 0.363 L (regression of S on L)

The results listed in Table II and represented in Figure 3 show the mean of the individual back-calculated lengths at ages that estimated by the three back-calculation formulae used in this study. Results of the ANOVA test indicated that there is no statistically significant difference between the mean lengths at ages that estimated by the three back-calculation methods for H. harid (F=0.0034, P=0.9966) and S. ferrugineus (F=0.0023, P=0.9977).


 

The mean calculated lengths at corresponding ages that estimated by the body proportional hypothesis (BPH) formula for males and females for each species (listed in Table II) were found to be not statistically significantly different (H. harid: F=0.003, P=0.954; S. ferrugineus: F=0.02, P=0.894). So, the growth in length and annual increment were determined from the mean calculated lengths at ages that estimated by the body proportional hypothesis (BPH) formula applied on pooled data (sexes combined) for each species and the results are represented in Figure 4.


 

The growth equation

Appling the Ford (1933) and Walford (1946) method to the mean lengths at ages estimated by the body proportional hypothesis (BPH) formula applied on pooled data, the growth parameters; L∞, K could be estimated and then the value of t0 was determined and thus the VBGE for describing the growth in length of both species could be written as follows: Lt = 54.0436 [1- e -0.168 (t+0.707)] for H. harid and Lt = 51.238 [1- e -0.170 (t+0.889)] for S. ferrugineus. The von bertalanffy growth curves are shown in Figure 5.

Length-weight relationship and growth in weight

Total fish lengths and their corresponding weights for H. harid and S. ferrugineus are represented in Figure 6. The length - weight relationship for both species could be described by the nonlinear (power) equations as follows:

For H. harid:

W = 0.026 L 2.81 (R2 = 0.967, n = 73 males)

W = 0.021 L 2.91 (R2 = 0.986, n =594 females)

W = 0.027 L 2.82 (R2 = 0.985, n = 667 both sexes)

For S. ferrugineus:

W = 0.020 L 2.973 (R2 = 0.987, n = 221 males)

W = 0.017 L 3.036 (R2 = 0.985, n = 295 females)

W = 0.020 L 2.970 (R2 = 0.989, n = 516 both sexes)


 

To check if the growth is isometric (having the value of the exponent ‘b’ equal to 3) or not, the estimated value of ‘b’ for males and females was tested if it was significantly different from 3 or not using the t-test of Pauly (1984a). The results of the t-test indicated that the growth of males and females of H. harid is negatively allometric where the ‘b’ values for both males and females are significantly smaller than the slope value ‘3’ of the isometric growth (for males: t = 3.0852, critical t value = 1.994 for P = 0.05; for females: t = 6.3257, critical t value = 1.965 for P = 0.05). For S. ferrugineus, the growth of males and females was revealed to be isometric, where the b values were found to be not significantly different from the slope ‘3’ of the cubic law of the isometric growth (for males: t = 1.171, critical t value = 1.972 for P = 0.05; for females: t = 1.6449, critical t value = 1.972 for P = 0.05)

Using the obtained equations describing the length - weight relationship for both species, the back-calculated lengths at ages were used to estimate their corresponding weights at ages and thus calculate the growth in weight for both species shown in Figure 7.



 

 

Length and age at first capture

Figure 8 shows the results concerning the probability of capture of both species H. harid (A) and S. ferrugineus (B) obtained by the cumulative method described by Pauly (1984b) in the FiSAT II software. The length at 50% probability of capture L50% or Lc and its corresponding age tc was found to be 19.33 cm and 1.93 year for H. harid and 19.30 cm and 1.88 year for S. ferrugineus.

Mortalities and current exploitation

The instantaneous total mortality coefficient Z for H. harid and S. ferrugineus was estimated as the absolute value of the slope of the right-hand descending line in Figure 9 (the linearized length-converted catch curve as obtained from the FiSAT II software). The value of Z was found to be 0.96 year-1 for H. harid and 0.77 year-1 for S. ferrugineus. The value of the natural mortality coefficient “M” was estimated as 0.294 year-1 for H. harid and 0.299 year-1 for S. ferrugineus. The fishing mortality coefficient F was estimated to be 0.670 year-1 for H. harid and 0.471 year-1 for S. ferrugineus. The current exploitation rate Ecur, which is expressed as the ratio of F/Z, was determined to be 0.69 year-1 for H. harid and 0.61 year-1 for S. ferrugineus.


 

Table III.- Parameters used to estimate the yield per recruit of H. harid and S. ferrugineus collected from Jeddah fisheries.

Parameter

H. harid

S. ferrugineus

K

0.168 year-1

0.170 year-1

W

2078.3 g

2390.7 g

t0

-0.7073 year

-0.8894 year

tc

1.93 year

1.88 year

tc

2.99 year

3.04 year

tr

0.85 year

0.77 year

M

0.294 year-1

0.299 year-1

Z

0.96 year-1

0.77 year-1

Fcur

0.67 year-1

0.471 year-1

F

Variable

Variable

 

Table IV.- The yield per recruit (Y/R) and biomass per recruit (B/R) corresponding to Fcur F0.1 and F max at two values of age at first capture of H. harid and S. ferrugineus in Jeddah fisheries.

Reference point

H. harid

S. ferrugineus

tc = 1.93 year

tc = 2.99 year

tc = 1.88 year

tc = 3.04 year

F (year-1)

Y/R (g)

B/R (g)

F (year-1)

Y/R (g)

B/R (g)

F (year-1)

Y/R (g)

B/R (g)

F (year-1)

Y/R (g)

B/R (g)

F 0.1

0.245

107.6

439.0

0.453

126.7

279.7

0.307

134.7

438.6

0.466

153.4

329.1

F max

0.401

112.5

280.6

0.686

129.0

188.0

0.434

137.7

317.2

0.830

158.1

190.4

F cur

0.670

108.1

161.4

0.670

129.0

192.5

0.471

137.6

292.1

0.471

153.6

326.0


 

Maximum yield and biomass per recruit

The parameters used to estimate the yield per recruit of H. harid and S. ferrugineus in Jeddah fisheries are given in Table III. The estimated Y/R and B/R as a function of fishing mortality at two values of age at first capture tc = 1.93 and 2.99 year for H. harid and tc =1.88 and 3.04 year for S. ferrugineus are shown in Figure 10.

The yield per recruit and biomass per recruit (in grams) corresponding to the current fishing mortality Fcur relative to those corresponding to the two biological reference points Fmax and F0.1 estimated at the two levels of age at first capture are listed in Table IV. For H. harid, at the current level of fishing mortality Fcur = 0.67 year-1 and age at first capture tc = 1.93 year, the yield and biomass per recruit were estimated to be 108.1 and 161.4 g, respectively. For S. ferrugineus, the current level of fishing mortality Fcur = 0.471 year-1 and age at first capture tc = 1.88 year, resulted in a yield per recruit of 137.6 g and a biomass per recruit of 292.1 g.

Increasing the age at first capture from 1.93 to 2.99 year (corresponding to 25 cm total length) for H. harid increased the yield and biomass per recruit at the current level of fishing mortality to 129.0 and 192.5 g, respectively. Similarly, when the age at first capture increased from 1.88 to 3.04 year (corresponding to 25 cm total length) for S. ferrugineus, the yield and biomass per recruit at the current level of fishing mortality increased to 153.6 and 326.0 g, respectively.

 

Discussion

Age determination and back-calculations

In the present study, the scales were used for age determination of H. harid and S. ferrugineus in Jeddah fisheries based on two criteria (Williams and Bedford, 1974): the recognized characteristic pattern shown on the scales (e.g., Fig. 11) and the annual time scale assigned for each pair of alternative opaque and hyaline bands (annulus) as confirmed in previous studies on scarid species (Warner and Downs, 1977; Lou, 1992; Fowler, 1995; Choat et al., 1996; Ali et al., 2011; Mehanna et al., 2014).

In the present study, the relationship between fish length and scale radius showed that the larger the observed fish length (L), the larger the scale radius at capture (S), based on the strong linear relationship: L = 2.901 S – 2.175 for H. harid and L = 2.564 S – 1.726 for S. ferrugineus. For H. harid ranging in total length from 12.4 to 48.2 cm, eleven annuli were laid down on the scales, while for S. ferrugineus ranging in total length from 13.2 to 48.5 cm, fourteen annuli could be observed on the scales.

The back-calculated lengths at ages that are estimated by the Fraser-Lee formula and the body proportional hypothesis (BPH) formula were almost the same for both species. This is because the two methods were based on the same linear regression (L on S), whereas the scale proportional hypothesis (SPH) formula produced slightly lower back-calculated lengths at ages because it is based on a different linear regression (S on L) as indicated by Francis (1990). However, results of the ANOVA test showed that the difference between the back-calculated lengths at ages estimated by the three different methods for the two species (pooled data) was not statistically significant. These results agree well with what is reported by Pierce et al. (1996).

The back-calculated lengths at ages that are estimated by the body proportional hypothesis (BPH) formula, as one of the two proportional hypotheses recommended by Francis (1990) for back-calculations, were considered in the present study to calculate the growth in length and growth parameters for sexes combined (pooled data) since there was no statistically significant difference between the back-calculated lengths at corresponding ages for males and females. Because the two scarid species are protogynous (females change to males in case of deficiency) (Ali et al., 2011; Choat and Robertson, 1975), females predominate in the younger length or age groups while being missed in older groups which are predominated by males.


 

Table V.- Growth parameters of H. harid and S. ferrugineus in the Red Sea estimated by different Authors.

Reference

H. harid

S. ferrugineus

L

K

t0

Ǿ

L

K

t0

Ǿ

Present study (Red Sea–Saudi Arabia)

54.0436

0.168

-0.7073

2.69

51.238

0.170

-0.8892

2.65

Ali et al (2011) (Red Sea- Saudi Arabia)

44.59

0.17

-1.52

2.53

61.4

0.1

-2.20

2.60

Mehanna et al (2014) (Red Sea- Egypt)

57.16

0.23

-0.69

2.88*

-

-

-

-

 

* Estimated from the available Land K values.

 

Table VI.- Length-weight relationship parameters of H. harid and S. ferrugineus in the Red Sea estimated by different Authors.

Reference

H. harid

S. ferrugineus

Length range

a

b

r2

Length range

a

b

r2

Present study (Red Sea–Saudi Arabia)

12.4 - 48.2 TL

0.027

2.82

0.985

13.2 - 48.5 TL

0.020

2.97

0.989

Ali et al (2011) (Red Sea- Saudi Arabia)

15.9 – 31.0 SL

0.023

2.99

-

12.5 – 31.6 SL

0.019

3.09

-

Mehanna et al. (2014) (Red Sea- Egypt)

17.0 – 50.0 TL

0.018

2.932

0.975

-

-

-

-

 

TL, total length; SL, standard length.

 

Growth in length

The growth rates estimated from the average back-calculated lengths at ages for sexes combined (pooled data) indicated that the maximum rate of growth in length was attained during the first year of life for both H. harid (13.82 cm) and S. ferrugineus (14.59 cm). During the second year, the annual increment was reduced to less than half of its value in the first year for H. harid (6.9 cm) and S. ferrugineus (6.5 cm), followed by gradual decrease during the next years. This trend of growth in length is more or less similar to those observed and reported in previous studies on the same two species in the Red Sea (Ali et al., 2011; Mehanna et al., 2014,) and other scarid species (Choat et al., 1996; Taylor and Choat, 2014).

Growth parameters

Considerable variability was found among the von Bertalanffy growth parameters estimated by different authors based on back-calculated lengths-at-ages, where scales were used for age determination for H. harid and S. ferrugineus in the Red Sea (Table V). This variability might be because of different back-calculated lengths at ages calculated using different back-calculation methods. Collecting scales from different body sites, measuring scales at different angles, and poor representation of all size groups in the samples may lead to a wide variation in the values of the regression analyses parameters used in the back-calculation methods (Carlander, 1982; Hirschhorn and Small, 1987).

For H. harid, the growth parameters estimated in the present study are comparable to those estimated by Mehanna et al (2014). The larger values of the asymptotic length and growth coefficient may be due to the slightly wider length range (17–50 cm) with fewer age groups assigned (8 age groups compared to 12 age groups in the present study).

For S. ferrugineus, Ali et al. (2011) estimated an extremely high asymptotic length (61.4 cm, standard length) with a corresponding very low growth coefficient (0.1 yr-1) compared to the maximum observed length they recorded (31.6 cm). In the present study, the estimated asymptotic length (51.2 cm total length) was close to the maximum observed length (48.5 cm) and much smaller than that estimated by Ali et al. (2011).

However, both values of the growth coefficient for H. harid (0.168 year-1) and S. ferrugineus (0.1704 year-1) indicate that both species are moderate growth species (Branstetter,1987). H. harid has slightly larger growth performance than that of S. ferrugineus due to the larger asymptotic length of the former (Table V).

Length-weight relationship

From the power equation describing the relationship between fish length and weight, the fish growth can be determined to be isometric or not. When the exponent ‘b’ equals ‘3’ then the growth is isometric. Values of ‘b’ lower than ‘3’ mean negative allometric growth, whereas b values higher than ‘3’ refer to positive allometric growth (Froese, 2006). As indicated in Table VI, the value of the exponent ‘ b ‘ for H. harid was significantly lower than ‘ 3 ‘, reflecting a negative allometric growth for this species in Jeddah fisheries.

Mehanna et al. (2014) predicted a negative allometric growth for the same species having comparable size range and length type (total length) in the Egyptian Red Sea. Ali et al. (2011) used smaller size range and length type (standard length) for the same species in Jeddah fisheries estimated the ‘b’ value to be 2.99 which is not significantly different from the isometric growth (b = 3). Also, Ali et al. (2011) estimated higher ‘b’ value for S. ferrugineus (3.09) compared to that found in the present study (2.97). However, Carlander (1977) showed that using the standard-length measurements results in higher condition factors than that when using measurements of larger length types (fork and total length). This, in addition to the narrow size range, may be the reason behind the larger ‘b’ values obtained by Ali et al. (2011) compared to that obtained in the present study.

Exploitation and maximum yield per recruit

The obtained results of mortality coefficients listed in Table III indicate that the stocks of both species are subject to a fishing mortality that is almost double the amount of natural deaths. To assess the stock status of both species under the current level of fishing mortality, the Beverton and Holt’s model (Beverton and Holt, 1957) was applied to estimate the yield per recruit at various levels of fishing mortality giving the yield per recruit curve for each species (Fig. 10).

For H. harid, it is evident from the results listed in Table IV and shown in Figure 10, that the current fishing mortality rate Fcur = 0.67 year-1 is higher than both values of Fmax = 0.401 year-1 and F0.1 = 0.245 year-1, which means that the stock of this species is currently overexploited. Increasing the age at first capture from 1.93 to 2.99 year increased the yield and biomass per recruit at the current level of fishing mortality from 108.1 and 161.4 g (12.4% of the virgin (un-fished) biomass per recruit ‘Bv/R=1305.29 g’) to 129.0 and 192.5 g (14.7% of Bv/R), respectively.

For S. ferrugineus, the current level of fishing mortality Fcur = 0.471 year-1 is higher than both values of Fmax = 0.434 year-1 and F0.1 = 0.307 year-1, reflecting a current overexploitation of the stock of this species in Jeddah fisheries. Similarly, when the age at first capture increased from 1.88 to 3.04 year, the yield and biomass per recruit at the current level of fishing mortality increased from 137.6 and 292.1 g (19.3% of Bv/R=1516.2 g) to 153.6 and 326.0 g (21.5% of Bv/R), respectively.

For both species, the levels of biomass per recruit at the current levels of fishing mortality may be not enough to provide sufficient recruitment to the fishery. Goodyear (1993) indicated that the recruitment process is dependent on the spawning stock biomass per recruit (SSBR). He also reported that the spawning potential ratio SPR = SSBRfished / SSBRunfished, which is proportional to B/Rfished / B/Runfished expressed in the Beverton and Holt’s yield per recruit model, should be between 20–30% to obtain the maximum or near maximum yield per recruit. Thus, we recommend reducing the current level of fishing mortality to that of the target reference point F0.1 = 0.453 year-1 after increasing the age at first capture from 1.93 year to 2.99 year to get a biomass per recruit of 279.7 g (21.4 %) for H. harid. For S. ferrugineus, we recommend reducing the fishing mortality from the current level to that of the target reference point F0.1 = 0.466 year-1 after increasing the age at first capture from 1.88 year to 3.04 year to get a biomass per recruit of 329.1 g (21.7 %).

However, increasing the age at first capture means improving the fishing gear selectivity to avoid the capture of young immature fish; by increasing the mesh size of the fishing net or/and avoiding the fishing ground having the young immature fish. Gabr and Mal (2016) recommended the use of trammel nets with 62 mm inner-panel mesh size to catch H. harid of a mean selection length of 24.46 cm total length, which is very close to that recommended in the present study to obtain the target yield and biomass per recruit.

 

Conclusion

Currently, the stocks of both species Hipposcarus harid and Scarus ferrugineus in Jeddah fisheries are overexploited. For both species, the levels of biomass per recruit at the current levels of fishing mortality may be not enough to provide sufficient recruitment to the fishery. Thus, we recommend reducing the current level of fishing mortality to that of the target reference point F0.1 = 0.453 year-1 after increasing the age at first capture from 1.93 year to 2.99 year to get a biomass per recruit of 279.7 g (21.4 % of the virgin stock biomass per recruit) for H. harid. For S. ferrugineus, we recommend reducing the fishing mortality from the current level to that of the target reference point F0.1 = 0.466 year-1 after increasing the age at first capture from 1.88 year to 3.04 year to get a biomass per recruit of 329.1 g (21.7 % of the virgin stock biomass per recruit).

 

Acknowledgments

This project was funded by the Deanship of Scientific Research (DSR), at King Abdulaziz University, Jeddah, under Grant No. 461/150/1434. The authors, therefore, acknowledge with thanks DSR for technical and financial support.

 

Statement of conflict of interest

The authors declare no conflict of interest.

 

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