Coral reef fisheries benchmarks in WIO

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and weighed life histories in 62 reefs in high compliance closures >15 years old along the ... virgin biomass with no variable picked as significant by the stepwise ...
Coral reef fisheries benchmarks in WIO By Tim McClanahan WCS

Introduc)on Evalua=ng the status of fisheries requires establishing benchmarks derived from unfished ecosystems. Reef fish biomass data were used to calculate a benchmark and weighed life histories in 62 reefs in high compliance closures >15 years old along the east African coastline. Characteris=cs of the biomass and life histories were classified and described for total, fishable, target, and non-target groups. Benchmark varia=on with 18 variables including habitat, number of species, life histories, and thermal and produc=vity environments was evaluated.

Results Fisheries target and non-target biomass were not sta=s=cally different from each other, sharing an equal propor=on of the total biomass, but different from total and fishable biomass . Total

Fishable

Target

Non-target

F-

biomass

biomass

biomass

biomass

value

1124 ± 451.8a

1050 ± 437.1a

590.3 ± 329.0b

466.4 ± 232.2b

48.57

0.0001

22.79 ± 2.21a

23.82 ± 2.17b

27.06 ± 2.27c

19.91 ± 2.11d

114.35

0.0001

Optimum length (cm)

25.47 ± 2.79a

26.73 ± 2.78a

30.91 ± 2.92b

21.69 ± 2.51c

119.39

0.0001

Maximum length (cm)

42.84 ± 4.5a

45.02 ± 4.43b

50.43 ± 5.03c

38.47 ± 4.749d

71.03

0.0001

Life span (yr)

9.50 ± 0.74a

9.83 ± 0.81a

10.75 ± 1.17b

8.71 ± 0.72c

57.66

0.0001

Generation time (yr)

2.97 ± 0.21a

3.04 ± 0.24a

3.31 ± 0.36a

2.71 ± 0.18b

32.8

0.0001

2.29 ± 0.16a

2.36 ± 0.17a

2.52 ± 0.26b

2.16 ± 0.16c

39.18

0.0001

Growth rate (cm yrˉ¹)

0.45 ± 0.04a

0.44 ± 0.04a,b

0.42 ± 0.06b

0.45 ± 0.04c

6.51

0.0001

Natural mortality (M)

0.95 ± 0.08a

0.90 ± 0.09a

0.83 ± 0.09b

1.00 ± 0.13c

34.38

0.0001

Trophic level

2.89 ± 0.18a

2.90 ± 0.20a

3.19 ± 0.29b

2.52 ± 0.16c

103.01

0.0001

Biomass category

Mean ± SD Length at first maturity (cm)

Age at first maturity (yr)

P-value

Life history metrics were poor predictors of total virgin biomass with no variable picked as significant by the stepwise regression using BIC criteria.

AIC Intercept Number of species

t-

F-

ratio

ratio

142.42 ± 274.43

3.57

0

0.61

20.95 ± 5.87

0.52

12.75

0.0007

Estimate (mean ± SE)

Nine variables were not significant following AIC.

P>|t|

VIF

1.0

R2

p>F

0.19

0.0007

Number of species was significant and posi=ve for all biomass categories and explained 19% and 14% of the variance in virgin and fishable, biomass, respec=vely.

Habitat and environmental associa=ons predicted 33% of the variance in the virgin biomass but indicated only a few significant variables. AIC Intercept Ocean exposure Multivariate stress model Erect algae

F-

Estimate (mean ± SE)

t-ratio

2128.23 ± 413.18

5.15

0

0.0001

-300.67 ± 81.35

-3.7

13.66

0.0008

1.2

-1178.97± 478.32

-2.46

6.08

0.019

1.1

-13.2 ± 6.33

-2.08

4.34

0.04

1.11

ratio

P>|t|

VIF

14 variables were not significant after conducting the step-wise regression analysis.

R2

p>F

0.33

0.003

Small decreases in biomass however may trigger a sequence of events leading to declines in key ecosystem processes (McClanahan et al., 2011).

The variability of maximum sustained yields increased from B0, to z, to r for the likely ranges of these values.

Conclusions and recommenda)ons Differences between total and fishable biomass were small but target and non-target biomass yields were around half the total yields Target biomass had larger body sizes, longer life span, lower natural mortality metrics, and higher trophic levels. Therefore, target fish would need to mature longer than non-target groups to achieve op=mum yields. Consequently, high effort should lead to increasing dominance of non-target groups with faster life histories, which may maintain produc=on but at the cost of reducing valuable stocks.

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