Abudance of demersal fish by longline fishing gear in Sentang Waters, Serdang Bedagai Regency, Sumatera Utara

Demersal fish is the main catch for traditional fishermen using longline fishing gear in Sentang Beach waters located in Serdang Bedagai Regency, North Sumatra Province. The high economic value of demersal fish is the focus of traditional fishermen to make it the main catch. This study aims to determine the composition of the catch and the abundance index of demersal fish caught by longline in the waters of Sentang Beach. The method used in this research is to use a random sampling method with the mechanism of joining fishermen to go to sea for one month every time fishermen go to sea in November 2022. The data analysis technique uses abundance index value analysis and multiple linear regression analysis. The results showed that the composition of demersal fish caught using longline fishing gear consisted of 5 species of fish namely Snapper (Lates calcarife), Threadfins (Eleutheronema tetradactylum), Pomfret (Pampus argenteus), Sagor catfish (Hexanematichthys sagor) and Mullet (Valamugil saheli) with the highest abundance index obtained by the mullet species. Obtained the highest relative relative abudance of fish found in Senangin species which is 100% and the lowest relative relative abudance of fish in the species of Snapper which is 1.14%.


Introduction
The waters of Sentang Village, Serdang Berdagai Regency have good potential to be utilized in the capture fisheries sector and the Serdang Bedagai coastal area offers excellent potential to be utilized as cultivation land, marine ecotourism and the fisheries sector [1].
Abundant biological resources plus the open access nature of the resources encourage people who use fish resources to use production as an indicator and target in fulfilling fishing business activities so that it is in accordance with the opinion of [2] Demersal fish resources in shallow waters are often the target of exploitation because of their relatively high selling value and also the ease of reaching fishing grounds [3].
Oceanographic conditions in a body of water influence fish behavior.Oceanographic parameters that play a direct role in fish movements include sea water temperature, salinity, currents and food availability.Temperature is the most important oceanographic parameter in predicting the presence of fish because each fish has a different optimum temperature for its life.If the temperature in the water matches the temperature needed by the fish, the fish tend to have a better appetite.Salinity has an influence on the ongoing biological processes which directly affect growth rate, amount of food consumed food conversion value and survival rate [4].
Most fishing operations in the Sialang Buah Serdang Bedagai waters are still carried out in a traditional (basic) manner, both in terms of the equipment used and the method of taking the fish, so that 2 the fishing operation area is 3-4 miles or less from the beach.The consequences of excessive use of regional resources include a decline in fisheries productivity, then developing issues regarding fishing which tend to increase.This is characterized by fishing competition which results in a decline in Demersal Fish production.Based on this, research is needed to collect data and knowledge that can be used to manage fisheries sustainably.This is the background for research on the species composition and abundance of demersal fish to be carried out.

Study Area
Fish data was collected every time a fisherman caught during November with 12 observation stations which can be seen on the research location map (figure 1).Each station carries out direct observations of water physical parameters in the form of sea surface temperature and salinity.

Procedures
The fishing gear used is a longline with a main line of 200 meters and 200 number 8 hooks.During fishing, the longline is lowered into the water, left for 30 minutes to 1 hour, and then pulled out.Fish sorting is carried out according to the type of fish caught when pulling the fishing line.Further, the types of demersal fish caught were identified according to [5][6][7][8][9].Demersal fish management strategies are formulated taking into account habitat, socio-economic conditions, catches, species and amount of fish caught, fishing equipment used and relevant fisheries regulations..The data that has been obtained is then processed by calculating the relative abudance of demersal fish and the relationship between catches and water physical factors where fish data collection is carried out simultaneously with assessing water physical parameters.

Regression analysis
The research data were analyzed using descriptive analysis which was then presented in graphical form.Next, a statistical analysis was carried out to see the relationship between sea surface temperature and the catch of julung fish.Analysis of the influence of SST and chlorophyll-a on catch abundance was carried out using multiple regression at a confidence level of 95%.In order to prevent bias, normality and multicollinearity tests are included.After going through the classical assumption test, you can continue with multiple regression to estimate the influence of the independent variable (X) on the dependent variable (Y).The following is the regression equation used Y = α + β1X1 + β2 X2 where: Y= Abundance of catch; α = constant (intercept) β1 = sea surface temperature regression coefficient, β2= brightness concentration regression coefficient, X1 = SST ( 0 C), X2 = brightness (mg/m3).

Composition of catch
From the composition of the total number of individuals caught in the Sentang waters of Serdang Berdagai Regency, North Sumatra with a total of 200 fishing rods no. 8, shows that the composition of the number of individuals caught is 1467 individuals.The highest composition is found at station 3 with a total of 590 fish individuals consisting of 3 species, while the lowest number of fish individuals obtained is found at station 7 with a total of 4 fish individuals consisting of 1 species.According to the findings of research conducted in Sentang Waters.Serdang Bedagai Regency at 12 stations obtained 5 species of fish with 5 different families too.The fish caught using longlines during November are Senangin, Belanak, Snapper, Kedukang and Pomfret.
Based on the results of research conducted at all research stations, it was found that the highest individual value was obtained by the mullet species with a total of 1004 individuals.This is because mullet fish generally live in groups on beaches or shallow waters which are around 300 meters to 1 mile from the shoreline.Mullet fish is one of the targets of fishermen's catch in Sentang Waters, Serdang Berdagai Regency.This is in accordance with [11] who stated that mullet fish form schools in shallow coastal waters, entering goba areas (lagoons), estuaries and fresh waters, to look for food.Juvenile mullet fish can enter mangrove waters, eating small algae, diatoms and detritus material on the bottom along with sand and mud.
Based on the results of research that has been carried out, the fewest species found from all stations are rock snapper, namely 2 individuals.The small number of snapper fish caught by longlines is because fishing activities are carried out during the day, while snapper fish are a nocturnal species that move actively to find food at night.This is in accordance with [12] who states that snapper fish are generally wild fish, predators that are always active in looking for food at night (nocturnal).Nocturnal fish activity is not as much as diurnal (daytime) fish.Their movements are slow, tend to be silent and their movement direction is not equipped with a large area compared to diurnal fish.

Relative abundance
Obtained the highest relative relative abudance of fish found in Senangin species at stations 1,5,7,8 and 12 which is 100% and the lowest relative relative abudance of fish in the species of Rock Snapper which is 1.14%.The high value of relative abundance at stations 1,5,7,8 and 12 is influenced by the catch results where the only catch at that time was senangin fish.Based on the results of the research, it is known that the fish with the highest relative abundance percentage value is obtained by Senangin fish of 100%.The high value of percentage abundance in senangin fish is influenced by the salinity of the waters.Senangin fish caught by rawai in the waters of the Sentang Coast with salinity ranging from 25.5 ppt to 28 ppt with a regression value of 0.82 or it can be said that 82 percent of salinity affects the abundance of senangin fish.This is in accordance with the statement [13] which states that Senagin fish can tolerate salt levels or salinity with a low salinity range between 25 -38 ppt.The lowest relative relative abudance was obtained by snapper at 1.14% at station III.The low relative relative abudance obtained by snapper was due to fishing activities carried out during the day.Snapper fish tend to be active at night because they prey not by sight but by their sense of taste.This is in accordance with [14] who stated that snapper fish are generally wild fish, because they are generally predators that are always active in looking for food at night (nocturnal).The activity of nocturnal fish is not as active as diurnal (daytime) fish.Their movements are slow, tend to be silent and their movement direction is not equipped with a large area compared to diurnal fish.

Relationship between demersal fish abundance and temperature and salinity
The physical parameters of Sentang waters such as temperature range between 29.02°C -33.02°C.The graph in Figure 6 shows that the water temperature at the research location tends to be stable from station 4 to station 12.The highest temperature at the research location is at station 3 with a value of 33.02°C, while the lowest water temperature is at station 8 with a value of 29.02°C.Based on the results of research conducted in Sentang Waters, Serdang Bedagai Regency, North Sumatra, it was found that temperature values at all stations ranged between 29.02°C-33.2°C.The highest water temperature at the research location was found at station 3 and the lowest water temperature at the research location was found at stations 4 and 8.The temperature differences at several stations are due to the influence of the high intensity of light entering the water column.In accordance with the statement [15] which states that the temperature value in a body of water can be influenced by solar radiation factors, the high intensity of solar radiation can cause a high level of heat absorption into the water and the decomposition process.Based on the results of research that has been carried out, the most fish caught by longline are at station 3 which has a temperature of 33.02°C.At this temperature, demersal fish in groups will easily adapt because the optimum temperature for fish is generally 28°-33°C.The ideal temperature range for fish life in tropical seas is between 28° to 33°C [16].
The highest salinity value is at station 3 with a value of 28.02‰, while the lowest salinity value is at station 8 with a value of 25.31‰.Overall, salinity values fluctuate.In general, the salinity value of sea water in the middle is 35%.Based on research results in Sentang coastal waters, salinity values ranging from 25.25 to 30.28 were obtained.The large difference in salinity values is influenced by the mixing of fresh water and brackish water on the beach.This is in accordance with [17] stated that high salinity is generally found in the middle of the sea, namely 35 ppt, while lower salinity is found in areas around the coast or in river estuary areas.The low salinity value found in Sentang Waters is thought to be due to the influence of rainfall that occurs during the west season.The research was carried out during the rainy season so that the salinity concentration decreased because it mixed with rainwater.This is in accordance with [15], who stated that water geography, tides, and rainfall all impact salinity.Low salinity affects fish abundance and diversity indices.This is because fish need optimal salinity for fish to osmosis.The lower the salinity, the smaller the osmotic pressure that affects biota according to [16] the optimal range of seawater salinity for fish is 30‰ -40‰.The results of the regression analysis of temperature on abundance are y = 0.7326x -20.64 with an R² value of 0.8859 which shows that temperature influences abundance by 88% and the rest is other factors.A positive regression value for temperature on abundance shows that if temperature increases by 1°C, abundance will increase by 0.7326.Based on the results of data analysis, it is known that temperature influences abundance by 0.8859.If we refer to the coefficient, correlation and interpretation relationship table [18] it is said that the temperature obtained during the research influences coastal waters very strongly with values ranging from 0.8-1.00,categorized as a relationship that very strong.The data shows that the highest relative abudance is found at a temperature of 30°C.It is suspected that 30°C is the temperature that demersal fish really like.This result is also in accordance with research [19] in his research stated that demersal fish like waters with an average temperature value of 29-30°C.The results of the regression analysis of salinity on abundance are y = 0.474x -12.22 with an R² value of 0.821 which shows that salinity influences abundance by 82% and the rest is influenced by other factors.A positive regression value on salinity indicates that if the salinity value increases by 1, abundance will increase by 0.474.Based on the analysis results, it was found that salinity affected abundance by 0.821.The relationship between salinity and abundance is in the very strong category with a correlation coefficient of 0.821.In accordance with the opinion expressed by [20] that the results of the interpretation of the correlation coefficient ranging from 0.75 -0.99 can show a relatively strong correlation relationship.This is in accordance with [21] Salinity has an important role and is closely related to the life of aquatic organisms including fish, where physiologically salinity is closely related to the adjustment of the fish's osmotic pressure.In general, demersal fish live in the salinity range of 32-34‰ in accordance with the statement of [20] which states that demersal fish like waters with an average salinity value of 32 -33.5‰.However, researchers obtained salinity results ranging from 25.6 -30.28‰, it is suspected that the low salinity was influenced by the region and season at the time the research was carried out.Rainfall also influences the level of salinity.The greater the rainfall in a sea area, the lower the salinity of the sea water and perhaps the most influential thing in this case is the supply of fresh water that flows from rivers around the coastal waters.

Conclusions
Obtained the highest relative relative abudance of fish found in Senangin species which is 100% and the lowest relative relative abudance of fish in the species of Rock Snapper which is 1.14%.According to the linear regression results, it was concluded that temperature and salinity parameters had an influence on catch results with the presentation values for each parameter being 88% and 82%.

Figure 2 .
Figure 2. Relative abundance of demersal fish in Sentang Waters.

Figure 3 .
Figure 3. Regression of temperature on abundance.The results of the regression analysis of temperature on abundance are y = 0.7326x -20.64 with an R² value of 0.8859 which shows that temperature influences abundance by 88% and the rest is other factors.A positive regression value for temperature on abundance shows that if temperature increases by 1°C, abundance will increase by 0.7326.Based on the results of data analysis, it is known that temperature influences abundance by 0.8859.If we refer to the coefficient, correlation and interpretation relationship table[18] it is said that the temperature obtained during the research influences coastal waters very strongly with values ranging from 0.8-1.00,categorized as a relationship that very strong.The data shows that the highest relative abudance is found at a temperature of 30°C.It is suspected that 30°C is the temperature that demersal fish really like.This result is also in accordance with research[19] in his research stated that demersal fish like waters with an average temperature value of 29-30°C.

Figure 4 .
Figure 4. Regression of salinity on abundance.The results of the regression analysis of salinity on abundance are y = 0.474x -12.22 with an R² value of 0.821 which shows that salinity influences abundance by 82% and the rest is influenced by other factors.A positive regression value on salinity indicates that if the salinity value increases by 1, abundance will increase by 0.474.Based on the analysis results, it was found that salinity affected abundance by 0.821.The relationship between salinity and abundance is in the very strong category with a correlation coefficient of 0.821.In accordance with the opinion expressed by[20] that the results of the interpretation of the correlation coefficient ranging from 0.75 -0.99 can show a relatively strong correlation relationship.This is in accordance with[21] Salinity has an important role and is closely related to the life of aquatic organisms including fish, where physiologically salinity is closely related

Table 1 .
Composition of catch.

Table 2 .
Abundance, temperature and salinity data table for each station.