Trap Net vs Electrofishing Sampling Methods

Bluegill Fish Populations: Sampling Methods Field research can often depend on the quality and reliability of the equipment used to record samples and observations. If researchers fail to understand their sampling measures and any potential limitations they present, the end result of the data collection and observations may not have the validity required to make realistic conclusions about the population in question.

Such is the case for studying fish populations, as different collection methods can skew the size and species of fish observed within a given sample area (Reynolds & Simpson, 1978). This current survey explores two sampling methods, trap-nets and electrofishing in order to highlight the potential limitations both show and to determine which of the graphs represent each method.

Ultimately, Gear B represents the electrofishing method because of the larger numbers of smaller fish, while Gear A represents trap-net methods because there are relatively no smaller fish present in the sample data. Trap-nets are a common collection method for research exploring biodiversity of fish in both fresh and saltwater environments. These are basically nets which trap fish species so that researchers can then count and measure the collected samples.

Prior use and research of the effectiveness of trap-nets in sampling methodologies has found that they do not typically sample smaller fish adequately (Laarman & Ryckman, 1982). Thus, they represent larger species of fish more, which can skew the observations to show that there are more populations of larger fish than smaller fish.

However, ultimately it is the size of the mesh that has the biggest impact on the size of the fish caught within the sample and so researchers must be aware of mesh size relative to the types of species within the field site environment. With larger mesh, more small fish can escape, skewing the data set to represent more larger fish within the sample collected. Additionally, electrofishing techniques were used within the same field site with different results.

Electrofishing techniques use charged electrodes in order to send an electrical current into the water to stun the fish, which can then be collected and measured. A direct current (DC) is pumped into the water and thus stuns any fish within its range. Fish are then collected with a dip net to be recorded. Mortality rates are often much lower compared to trap-net collection, but these methods can only be used in relatively small bodies of water, as the electrical charges can only travel so far.

The rate and size of the pulse impacts the type and size of fish being stunned. A higher pulse charge is needed to effectively stun larger fish. Yet, electrofishing has been proven to be more efficient at collecting a wider range of fish sizes and types. Prior research shows that there are more smaller fish represented in sample sets where electrofishing is the primary collection methodology (Reynolds, 1996). With this in mind, one can discern which methods were used in each of the gear sets.

All samples using both methods were collected towards the end of the month of May when the water temperature was about 23 degrees C. The trap nets had 1.2x1.5 m frames, dual throats, and 19-mm bar mesh. Moreover, the electrofishing was conducted at night and used pulsed DC electricity at approximately 250 V and 8A. Gear A is most likely a sample collected using electrofishing methods. There are much more smaller fish represented in the sample compared to Gear B, which their lengths varying at a much higher degree.