Canadian scientists tested nets with the bluegill, a fish often caught by anglers in North America. A control group were hooked and held out of the water, but not netted.
After the fish were held in a landing net for just 30 seconds, there was abrasion of the pectoral and caudal fins. There was also loss of scales and disturbance of the mucus layer. This layer normally protects the fish against infections, such as those from fungus.
No control fish died during a 168-hour holding period. However, 14% of the netted fish later died. It took between 2 and 4 days for them to loose their lives, which could give anglers the false impression that they are not the killers. The dying fish were unable to swim properly in their last day alive. There was finally such extreme erosion to their caudal fins that there was fungal infection covering up to 15% of the entire body.
All of the nets tested - rubber, knotless nylon, fine knotted nylon, coarse knotted nylon - caused fish deaths, with fine and knotted being the most deadly.
"In most cases, fish manifest few obvious immediate effects of the netting process, and when released, swim away in seemingly good condition. This leads anglers and managers to conclude that the total costs of the angling interaction for the fish consist of the energy expended in the struggle, the sub-lethal physiological disturbance from exercise, and associated hooking injury"1. Saprolegnian - a fungal infection
"To test this hypothesis, we used bluegill as a model species to examine the effects of different net mesh types (rubber, knotless nylon, fine knotted nylon, coarse knotted nylon) on injury and mortality following angling at 26°C."
"A control group consisted of individuals that were angled and held out of the water but not netted. Retention in a landing net for 30 s resulted in increased pectoral and caudal fin abrasion relative to control fish."
"Furthermore, evidence of dermal disturbance (i.e. scale and/or mucous loss) was more prevalent in netted fish than in control individuals. No control fish died during a 168 hours holding period, whereas mortality rates ranged from 4 to 14% for fish landed with nets, and the majority of mortality occurred between 48 and 96 hours post-treatment."
"Fish that died exhibited impaired swimming behavior for approximately 24 hours prior to death that was attributable to the extreme caudal fin erosion. Fish that died also had Saprolegnian¹ lesions on the caudal peduncle that had begun to progress anteriorly toward the gills."
"Our results indicate that fish captured and landed by hand had lower injury rates than those fish landed using a net and experienced no mortality. Conversely, all net types resulted in heightened injury and mortality with the knotted mesh types being more injurious than the rubber or knotless mesh."
"Fish landed with knotless mesh and rubber mesh exhibited mortality rates of 6 and 4%, respectively. The highest levels of mortality were observed in fine knotted (14%) and coarse knotted (10%) mesh types. All mortality occurred between 48 and 120 hours post-treatment."
"Saprolegnian¹ lesions were present on all fish that died and covered between 5 and 15% of the body of moribund fish. All fish that died exhibited extreme levels of caudal fin abrasion. In fact, most of the caudal fins had eroded to the caudal peduncle."
"All dead fish exhibited loss of scales and mucous and were observed to swim erratically for a period of ~ 12 hours prior to succumbing to their injuries."
"The mucous layer covering the dermal surface of the fish acts as a physical barrier to colonization by foreign organisms and possesses anti-fungal properties. Following an incident of dermal disturbance and infection, the fungal lesions spread across the surface of the fish. Saprolegnian lesions were present on all fish that died in our study and covered 5–15% of their bodies. The knotted mesh types have a higher incidence of dermal disturbance (i.e. loss of mucous and scales) that apparently promote fungal infection in netted fish."