First approach to ageing the cuttlefish Sepia bertheloti. Comparison of growth in two fishing areas

Abstract

The African cuttlefish Sepia bertheloti is fished on the West African coast, often mixed with S. officinalis and/or S. hierredda. As in other marine fished species, the sustainable management of their populations begins with the understanding of the species life-cycle. Wild-caught samples of S. bertheloti from the same season and two different fishing areas of West Africa were analysed: Morocco in the north and Guinea-Bissau in the south. This study aims to analyse, for the first time, the age and growth of the African cuttlefish by using the rostrum sagittal sections (RSS) of the Lower Beak. A total of 120 specimens were sampled in the laboratory (60 specimens from North Morocco and 60 specimens from Guinea-Bissau, with Dorsal Mantle Length (DML) ranged between 63-130 mm and 122-170 mm, respectively. DML, wet weight, sex and maturity state were recorded for each specimen. After extraction and preparation of hard structures, a constant pattern of increments was observed in the RSS of lower beaks. The number of increments was counted according to Perales-Raya et al. (20101, 20142). Individuals from Morocco showed 149-419 increments in beaks, while in those from Guinea-Bissau it was 174-433. Assuming a daily deposition of growth increments in RSS beaks (as has been validated in other cephalopod species), the results show different growth patterns in both areas of NW Africa. Sepia bertheloti shows a slower growth rhythm in the waters of North Morocco, with smaller DML at the same age than off Guinea-Bissau.

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First approach to ageing the cuttlefish Sepia bertheloti.
Comparison of growth in two fishing areas
Airam Guerra Marrero1*, Aurora Bartolomé2, David Jiménez Alvarado1, Lorena
Couce Montero1, Ana Espino Ruano1, José J. Castro1and Catalina Perales-Raya2
INTRODUCTION
RESULTS
1Biodiversity and Conservation Group (BIOCON), IU-ECOAQUA, University of Las Palmas de Gran Canaria, Canary Islands, Spain
2Centro Oceanográfico de Canarias (IEO-CSIC), Sta. Cruz de Tenerife, Spain.
* airam.guerra@ulpgc.es
The African cuttlefish Sepia bertheloti (Orbigny, 1839), is distributed throughout West Africa (Jereb and Ropper,
2005; Guerra-Marrero et al., 2019) that can reach sizes of 175 mm for males and 130 mm for females (Okutani,
1980). It is a species captured by trawling nets at depths between 70-140 m (Roper et al.1984), with a greater
presence in the fisheries from Sahara to Guinea-Bissau areas. The species is caught as by-catch in these fisheries,
where the main target species of cuttlefish are Sepia officinalis and Sepia hierreda in the northern and southern
waters respectively (Guerra et al., 2014).
At present, there is no information about its age and growth for the specie. In this study we compare the populations
of two West African areas: Morocco in the North and Guinea-Bissau in the south. Given the difficulties in estimating
de age of this species in structures such as the statolith, this study aims to analyse the age and growth of the African
cuttlefish by using beaks, where preliminary results in other cuttlefishes suggest a daily deposition (Lishchenko et al.,
2018)
A total of 1123 individuals of Sepia bertheloti were
collected from June 2018 to January 2020. They were
caught by commercial trawlers operating in Morocco (552
individuals) and Guinea-Bissau (571 individuals). See
Figure 1.
A subsample of 78 individuals from Morocco and 128 from
Guinea-Bissau were analysed. The subsample was
randomly performed by categorizing the individuals by
size range for both sexes every 5 mm of Dorsal Mantle
Length (DML).
The beaks were extracted, cleaned and stored in distilled
water at a temperature of 4ºC, according to the procedure
described by Perales-Raya et al. (2014).
The methodology used for processing the Lower beaks
was that described by Perales-Raya et al. (2010). Once
the beaks were processed, they were analysed using a
Nikon Microscope Multizoom AZ100 with and UV-
epiillumination attachment (vertical reflected light) (Fig. 2).
The increments observed were counted twice by the same
Figure 1. Sampling areas (FAO Fishing Areas) where the commercial
trawlers caught Sepia bertheloti in Morocco (Tangier zone) (FAO
1.11)and Guinea-Bissau (FAO 3.13). Exclusive Economic Zone
(EZZ) for Morocco (Tangier zone) and Guinea-Bissau in the FAO
Fishing areas are shaded
MATERIAL AND METHODS
Figure 2. Appearance of growth increments in the lower beak section of Sepia bertheloti (300x).
Reliable readings were obtained for 183 of 206 beaks analysed. Twenty-three beaks (11.17%) were discarded
because the structure had malformations (Fig. 3) or were severely damaged during grinding.
Cuttlefishes from Guinea-Bissau showed an estimated age between 94 (72 mm DML) and 433 days (160 mm
DML), with a mean age of 231.19 days Estimated age for individuals from Morocco ranged from 111 (60 mm
DML) to 419 days (140 mm DML), with a mean value of 220.53 days.
According to the AIC parameters (Table 1), the Schnute’s and exponential models were those that best
described the growth pattern of males and females, respectively, in the Morocco population (Fig, 4). Exponential
model, in turn, was the best one to describe the growth of the entire population (all individuals, Fig.4). In the
population of Guinea-Bissau, the von Bertalanffy model was the best describing the growth pattern in males
while the exponential model was the best for females (Fig. 4). For all the individuals, the Exponential model was
the best fitted (Fig. 4).
Statistically significant differences in growth rates (G and AGR) between sexes (t-test, p<0.0001) and areas (t-
test, p<0.0001) were found. The individuals from Guinea-Bissau showed a higher instantaneous relative growth
rate than the individuals from Morocco, showing faster growth at the same age (Table 2)
Figure 4. Best growth models fitted to Dorsal Mantle Length at age data for females, males and all individuals
of Sepia bertheloti caught in Guinea Bissau (Top) and Morocco (down).
Figure 3. Beak discarded from the analysis due to malformations
CONCLUSIONS
Table 1. Statistical parameters of different growth model fitted to Sepia bertheloti. Dorsal
Mantle Length-age data for Morocco and Guinea-Bissau. AIC: Akaike’s information
criterion, AICw: Akaike weight, BIC: Bayesian Information Criterion. Best model fit is
given in bold underlined.
trained reader. Coefficient of variation (CV) was used to estimate the precision of the readings and the
reproducibility of the method. According to Campana et al. (2001), to avoid any bias, the CVs were
averaged by age classes and CV<7,6% were take as valid, rejecting reads with CV> 7,6%. Growth
patterns and growth models were analysed according to Forsythe and Van Heukelem (1987) and Bolser
et al. (2018).
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ACKNOWLEDGEMENTS
Table 2. Dorsal mantle length growth-rates for each age-class of Sepia bertheloti females
and males from Morocco and Guinea-Bissau. G: instantaneous relative growth rate (%
DML d-1); AGR: absolute growth rate (mm d-1). X̄: average; SD: standard deviation.
AGM was supported by a grant of the University of las Palmas de Gran Canaria (ULPGC) with a research staff
training contract (PIFULPGC-2017-CIENCIAS-2).
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- Assuming the initial hypothesis of 1 increment – 1 day of life, a maximum life expectancy of 433 days was
obtained for Guinea-Bissau and 419 days for Morocco.
- Specimens from Guinea-Bissau showed larger sizes than individuals from Morocco for the same age
classes.
- For both areas, the best model that describes the growth of all individuals is the exponential. It was also
shown that males have a higher growth rate than females, and in turn, the population of Guinea Bissau was
the one that presented the highest growth rate.
- The differences in growth patterns seem a priori to be related to the different oceanographic conditions of
both areas (individuals from Guinea Bissau grow faster due to its warmer waters).