Genetic identification and diversity of stocks of the African bonytongue, Heterotis niloticus (Osteoglossiformes: Arapaiminae), in Nigeria, West Africa

Abstract

Inland fisheries are an important source of protein and income for people in Africa. Their sustainable management can greatly benefit from identification of regional genetic stocks and characterization of their genetic diversity, but such information is lacking for most African freshwater fisheries. The African bonytongue, Heterotis niloticus, is an important component of inland fisheries in West Africa. Nigeria has the largest fishery for African bonytongues, representing ~ 86% of the global total. Recent declines in yields at some Nigerian locations, however, suggest current levels of exploitation may be unsustainable. Habitat degradation also may be impacting some stocks. Despite its commercial and nutritional importance, the African bonytongue has been the subject of scant genetic research to support management. We examined patterns of genetic diversity in natural populations of H. niloticus at four locations in Nigeria, including Kainji Lake, a reservoir on the Niger River in north-central Nigeria, and three southern localities (Ethiope River, Igbokoda River, and Epe Lagoon), as well fish from the Ouémé River delta near Porto Novo, Benin. Eighty-five specimens were genotyped for nine microsatellite-loci. Genetic diversity estimates were highest at Kainji Lake, and substantially lower at southern localities. High levels of genetic differentiation were detected between samples from Kainji Lake and those from southern localities. Low, yet significant F_ST values were observed among samples from southern Nigerian localities that were more differentiated from the sample from nearby coastal Benin. We thus recommend that African bonytongues from the five locations be considered distinct genetic stocks and managed accordingly.

Figure 1

Map of the study area. Yellow stars indicate the collection sites of Heterotis niloticus for this study: Kainji Lake (10° 42′ N; 4° 42′ E and 9° 50′ N; 4° 37′ E); Ethiope River (5° 53′ N; 5° 43′ E), Igbokoda River (6° 17′ N; 4° 49′ E); Epe Lagoon (6° 34′ N; 3° 59′ E); Porto-Novo (6° 27′ N; 2° 37′ E). White circles show collection sites of H. niloticus in the study of Hurtado et al. (2013) in Benin.

Figure 2

Cross-validated DAPC scatter plots (dots represent individuals) using sampling locations as a priori groups: (a) including all individuals from the five sampling locations (30 PCs); (b) including only the individuals from the southern populations (20 PCs).

Figure 3

STRUCTURE bar plots of posterior probability for individual assignments using the admixture-correlated model for K = 2 to K = 5: (a) including all individuals from the five sampling locations; (b) excluding individuals from Kainji Lake.

Figure 4

Principal coordinate analyses (PCoA) based on pairwise genetic distances of individual multilocus genotypes (individuals are color-coded by sampled population and plotted on the first two coordinates): (a) including all individuals from the five sampling locations; (b) excluding individuals from Kainji Lake.

Figure 5

Isolation by distance (IBD) analyses. Correlation between geographic distance and genetic divergence F_ST for: (a) all five sampling locations; (b) the four southern sampling locations (i.e., excluding Kainji Lake); (c) considering all 83 individuals sampled separately; and (d) considering the 60 individuals from the southern sampling locations separately.