Morphometrics and pharyngeal characteristics of farmed and wild Nile tilapia

Abstract

While wild capture production continues to decline, fish farming is fast-growing in Uganda, with a substantial increase in the number of aquaculture farms. However, uncontrolled aquaculture can indirectly increase the damage on capture fisheries. For example, the juvenile wild fish can be harvested and marketed as farmed leading to stock decline. There is no size restriction at which farmed fish is harvested. This study compared specific morphological parameters in farmed and wild fish with intentions of developing a quick and cheap method of differentiating them. Data were compared in farmed and wild fish of relatively the same size, collected from Lukonda bay of Lake Victoria and analyzed following standard procedures. The results revealed higher values of morphological traits in the wild than in the farmed fish, averagely exceeding by 17%. However, variation in many of the studied parameters in wild fish were less explained by total length compared to the farmed. For example, total length explained only 29% of the variation in Anal fin length in wild fish compared to 49% in farmed (R²= 0.29 and 0.49, respectively). Regressing all parameters against total length showed coefficient of determination (R²) between 0.29 and 0.89 in wild and from 0.49 to 0.98 in farmed fish. In regards to length-weight relationship, wild fish exhibited negative allometric growth (i.e., b value of 0.30), as farmed positive allometric growth (i.e., b value equal to 3.121). As expected, the pharyngeal jaw bone development in wild fish was higher than in the farmed. Evidently, this study provides baseline information against which law enforcers can operate to mitigate illegal fishing in guise of marketing farmed fish. The length-weight relationship can adequately be used to differentiate the wild from farmed fish since the results were distinct in the two populations. Furthermore, the law enforcers can establish the ratio of different morphometric parameters for fish of a known source by those of a sample whose origin is being investigated. The corresponding ratio value reflects the source of fish which helps in differentiating wild from farmed fish. Hence morphometric differences can be used by law enforcers to differentiate between wild and farmed fish of comparable sizes and hence curb illegal capture of juvenile wild fish.