Effect of Net Colours on Efficiency of Monofilament Gillnets for Catching Some Fish Species in the Lake Volta at Kpong

Introduction

The significant growth in fish consumption has improved people’s diets around the world through different and nutritious food. In 2013, fish contributed to about 17 percent of the global population’s consumption of animal protein and 6.7 percent of all protein consumed. Moreover, fish provided more than 3.1 billion people with almost 20 percent of their average per capita intake of animal protein [1]. Apart from being a rich source of easily assimilated food, high quality proteins containing all essential amino acids, fish provides essential fats (e.g. long chain omega-3 fatty acids), vitamins (D, A and B) and minerals (including calcium, iodine, zinc, iron, and selenium) particularly if eaten whole [1]. Even small amounts of fish can have a significant positive nutritional impact on plant-based diets, and this is the case in many Low Income Food Deficient Countries (LIFDCs) and least- developed countries. Fish is usually high in unsaturated fats and provides health benefits in protection against cardiovascular diseases. It also helps in the development of the brain and nervous system of foetus and infants [1].

Many gears such as gillnets, traps and hook and lines are used in capture fisheries with the most common type being the gillnet [2]. A monofilament gillnet is one which is made of single strands of a synthetic material that looks like a strand of modern fishing line [3]. Gillnets are normally a rectangular sheet of netting which is hanged in a water body like a curtain to trap or catch fish by the gills (operculae) as they try to swim through the mesh of the net [4].

The term “gilling” refers to the way fishes gill cover may act as barbs to prevent them from escaping after been trapped in the meshes of nets. Gillnets are commonly used in the coastal and inland fisheries of the world because of their flexibility, low cost, and ease of operation [5]. Gillnet is one of the oldest types of fishing gear and is widely used to harvest different marine and freshwater species [6]. Gillnets are known passive gears, consisting of a large wall of netting which can be set at or below the surface on the sea bed, or any depth in between [7]. Its construction can be single, double or triple (trammel net) netting. Depending on the operation, a gillnet can be drifting, fixed or encircling. Fish caught in gillnets are usually gilled, but can be wedged, snagged or entangled [8, 9, 10].

Nets are made up of a single or multiple layers of monofilament mesh in which the fish is trapped in its gill, wedged when it is caught by the largest part of the body or entangled when the fish is held in the net by teeth, maxillaries, fins or other projections, without necessarily penetrating the mesh [11].

Visibility of the nets depends on net colour, the tone contrast with the background and species of fish, which might be affected by the time of day, and the seasonal changes in water clarity or colour [12]. Fish can differentiate between colours, hence different coloured nets may show significant differences in catches [13]. Gillnets are made with fine twines that are chosen to be relatively invisible to the fish, particularly at low light intensities. When a net is invisible, the target fish are unaware of its presence and hence swim into it, and may become trapped by the meshes [14].

Many fishermen use different methods and techniques both modern and traditional to increase their catch which comes with a cost either financially or sometimes leads to environmental degradation. Also, considerable research has been carried out to reduce bycatch in many of the world’s fisheries. Most of this work has been concentrated on trawled gears (e.g. Trawl, dredges, etc.) and less attention has been directed towards reducing bycatch in immobile gears such as gillnets [15]. Of the most existing gillnet studies, most have been concentrated on knowing the effects of biotic (behavior, morphology, and vertical distribution of fish) [15] and abiotic factors (eg. hanging ratio, thickness of monofilament , and mesh size) [10, 16] on the selectivity in order to increase catches of the targeted species. In contrast, few studies have investigated simple changes to the technical aspects (other than mesh size) as a tool for reducing bycatch [17].

Specifically, the effect of net colour on the selectivity and catchability of gillnets has been addressed by [14]. Fishermen on the lake Volta at Kpong and other parts of the world use different colours of gillnets not knowing which colour would increase their catch since particular notice has not been taken. It is in this vain that this work was carried out in order to know which net colour attracts more fish or is more invisible to the fish or a particular species of fish in order to reduce cost and degradation as a result of efforts to increase catch. This study is therefore aimed at identifying the type and number of fish species captured by each net colour as well as determine the morphometric characteristics of fish species caught by each net [18].

Discussion

Fish have different reactions against different net colours which may affect the efficiency of the nets [13, 16] however, this has not been fully investigated. In this study, green net caught the highest number of species and fishes which may be due to masking of the colour of the net with that of the water as a result of the growth of green algae in the water (less visibility to the fish) where this experiment was conducted. Similar to our finding, [19] concluded from their study that green net caught the most species hence has the best economic efficiency [20] also documented green net to be effective in capturing cod in the west part of Baltic Sea. However, lower catching efficiency was recorded by both dark and light green nets when compared to red, yellow, brown, and blue gillnets in Turkey (lake Beyşehir) [21]. The white net caught the next highest number of fish species followed by the brown net. This contradicts the findings of [22] where the white net caught the highest fish species in inland waters of Burla, India. The least catch recorded by the brown net may be due to its inability to blend well with the water, thus it was avoided by most fish species.

P. obscura, C. guntheri and S. schall were caught by only the green net which might be because the fish detected the other net colours except for the green hence they unknowingly tried swimming through the mesh of the green net where they were trapped. Species such as M. anguilloides, M. senegalensis, and G. niloticus were caught by only the green and brown nets. M. breviceps was caught by brown net only which could be attributed to the attraction of the fish to the colour brown. As stated by the reaction of fish to net colours varies with different fish species, which could be the reason for the different coloured nets capturing different fish species [16].

Conclusion

In conclusion, the green net had the highest catching efficiency followed by white and brown nets. From Table 1, M. abadii was the most specie captured in this experiment while P. obscura, P. senegalus, S. bastiani, S. schall, M. breviceps, and C. guntheri were the least. P. obscura, C. guntheri, and S. schall were captured by green net only, C. gariepinus, T. zillii, and S. bastiani was captured by white net only and M. breviceps was captured by brown net only.

From this study, we recommend that more research should be conducted on colour efficiency of gillnets to enhance the efficiency of fishermen. Also, other aspects such as the effects of hanging ratio and thickness on the efficiency of the monofilament gillnets should be investigated. We also recommend that the study period of this experiment be extended in order to establish a year-long efficiency of the different net colours.