Pot Entrance Research Articles

Simple pot modification improves catch efficiency and species composition in a tropical estuary mud crab (Scylla serrata) fishery

Pots are widely used fishing gear type for targeting different crustacean and fish species. Pot entrance size and design are among the most important technical parameters that influence the catch efficiency of certain species. An optimal pot entrance design should allow an efficient entry for the target species while preventing subsequent escape. The tropical estuary pot fishery targeting mud crab (Scylla serrata) in Vembanad Lake, India, employs rectangular pots with rectangular-shaped entrances. Low catch rates for target species and high bycatch rates are observed in this fishery. This study was carried out to investigate if a simple pot modification by extending the entrance of the traditional pots, can improve the catch efficiency of mud crab. Further, we estimated and compared the catch composition in this small-scale fishery using the traditional and modified entrance pots. The results showed that the catch efficiency for all sizes of mud crab is on average more than six times higher with the modified entrance pots compared to the traditional pots (622% (CI: 344–1867%)). However, significant quantities of juvenile crabs are caught in modified pots. Further, the bycatch ratio was significantly reduced for modified compared to the standard entrance pots in this fishery. These results show that such pot modifications have potential to significantly improve the catches in mud crab pot fisheries without increase in capture of bycatch species. However, additional mechanisms for excluding undersized crabs from pot catches should be investigated.

Improving size selection in the Norwegian red king crab (Paralithodes camtschaticus) fishery through modification to pot design and soak time.

Red king crab (Paralithodes camtschaticus) represent a valuable inshore fisheries resource for communities in northern Norway. The fishery is regulated east of the 26th meridian by vessel quotas and a minimum landing size of 130 mm carapace length. It is executed using baited pots that must be fitted with circular escape openings. Despite this, pot catches typically contain a large proportion of sub-legal sized crab. This implies increased labor-intensive sorting which can lead to crab becoming injured, loosing limbs or delayed mortality. Such consequences negatively impact the sustainability of the fishery. In this study, we examined the potential of two new pot designs and longer soak times to reduce undersized crab catches in comparative field trials. A design consisting of a tarpaulin panel on the pot entrance (intended to prevent undersized individuals from gaining traction and entering) was found to be no better at reducing undersized catches than the currently used commercial design fitted with escape openings. A second design with a baited lower chamber (which could be accessed through the pot floor via escape openings) was intended to motivate undersized crab to escape more readily. If fished commercially, any crab in the bottomless lower chamber would be left behind on the seabed upon hauling. This design was found to have minimal effects on sublegal catches. However, increasing the soak time of the currently used commercial design from 1 day to 4 days reduced undersized crab catches substantially. This effect is likely related to bait becoming exhausted over time, thereby increasing the opportunity and motivation of sublegal crab to find and exit the pot through the mandatory escape openings. Laboratory-based behavioural observations using the panel design indicated that entry was dependent on crab orientation and that crab were less successful at entering than for the commercial design. However, all sizes of crab used an unintentional fold in the panel to increase their entrance probability. This may explain the lack of success of this design during the field trials. We conclude that ensuring bait is exhausted by employing longer soak times than is typically used by fishers would reduce catches of undersized crab in the fishery. Data availability statementThe data underlying this article will be shared on reasonable request to the corresponding author.

Understanding and predicting the effect of entrance cone diameters on the catch efficiency of snow crabs (Chionoecetes opilio) in conical pots

Conical pots with a plastic entrance cone on the top are a type of fishing gear used to harvest snow crabs (Chionoecetes opilio) in Arctic regions. In this study, we assessed the effect of pot entrance diameter on the catch efficiency of snow crabs. We used catch data collected during sea trials of pots with different entrance cone diameters, and we conducted laboratory experiments where morphological characteristics of snow crabs were investigated to estimate the extent of the different body parts of the animal that are decisive for the catch efficiency of the pot entrance. The results show that body parts larger than the carapace affected pot entrance efficiency. We predicted that entrance diameter is a key factor determining catch efficiency and that the experimental pots used in this study would experience significant reductions in the catch of marketable size crab, compared to pots currently used in the fishery.

Using an innovative net-pen-based observation method to assess and compare fish pot-entrance catch efficiency for Atlantic cod (Gadus morhua)

In many places, gillnet fishing is considered a conservation threat for air-breathing marine species. Fish pots represent an alternative to gillnetting; however, due to their low catch efficiency pots are rarely taken up by commercial fisheries. To improve pot efficiency for Atlantic cod (Gadus morhua), we used a novel enclosure to observe cod interacting with pot entrances, and investigated several entrance design parameters, including funnel colour, entrance funnel presence, length and entrance form. We demonstrate that the key factor for entrance passage is to give cod an unobstructed view of the inside or outside when they try to enter or exit the pot, respectively. Funnel colour (colours tested: white, green and transparent) influences entrance passage rates, with significantly higher entrance passage rates for the transparent funnel. Funnel presence increases the entrance encounter rate by enlarging the outer opening of the entrance. It decreases exit rates by deflecting cod away from the inner entrance opening and by reducing the area in which the exit is perceptible to cod inside the pot. Increasing funnel length further reduces this area and may deter cod by the longer passage length. This is the first study to observe cod–pot interactions day and night using an infrared camera, revealing a pronounced diurnal pattern with few nocturnal entrance passages, suggesting that cod–pot interactions are primarily guided by vision. The findings underline the importance of funnels and reveals promising avenues for their further improvement, e.g., by using transparent fish retention devices. The new pen-based method is superior in several ways to conventional field-pot catch-rate comparisons: It allows identification of differences in catch efficiency and describes the underlying cod behavioural mechanism leading to these differences. Thus, it allows targeted, efficient and iterative cod-pot catch-efficiency enhancements.

Determining the fundamental metrics of pot entrances for New Zealand scampi, Metanephrops challengeri

Bottom trawling is a fishing method that typically produces high levels of bycatch and is associated with benthic habitat damage. Pots (also known as creels or traps) are used as a less impactful alternative to trawling in a range of crustacean fisheries and has been proposed as a method to harvest New Zealand scampi (Metanephrops challengeri). However, pots designed in Europe for the ecologically similar Norway lobster (Nephrops norvegicus) failed to capture scampi at an economically viable rate. Therefore, a series of laboratory experiments were conducted to investigate the behaviour of scampi around pots and to identify the drivers of successful entries. These experiments were conducted in flume tanks with the entry performance of standard and modified designs contrasted with a control. Scampi were significantly more likely to enter a control pot than either a standard or modified pot entrances. The width of the entranceway was the biggest hinderance to scampi entering pots, as further experiments showed that scampi were significantly more likely to enter the control (15 cm wide) or 12.5 cm entrances than 7.5 cm entrance. The height of the entrances had no impact as the control, 12.5 cm, 10 cm and 7.5 cm high entrances had similar likelihoods of success. Lastly the angle of the ramp also had a significant effect on the likelihood of scampi entering the pots, with a 20° ramp having a higher likelihood of success than the 30°,40° and 60° ramps These results show that scampi require wide, gently sloping entrances to consistently enter pots. Consequently, for a scampi potting fishery to be successful, NZ scampi-specific pot designs should be developed and tested.

Effect of pot design on the catch efficiency of snow crabs (Chionoecetes opilio) in the Barents Sea fishery.

The snow crab (Chioneocetes opilio) fishery in the Barents Sea is carried out by large offshore vessels, as the fishing grounds are located far from shore and the gear must be transported back and forth over long distances. Therefore, fishers use stackable conical pots that allow large numbers of pots to be carried on deck for each trip. One of the drawbacks of using stackable pots is that the entrance is at the vertex of the conical pot, which fishers claim does not provide the desired fishing efficiency. Thus, the goal of this study was to determine whether a different pot design would improve the catch efficiency of snow crabs. We investigated the efficiency of a new type of pot called the moon pot, which provides continuous increased bait odour intensity as snow crabs make their way towards the entrance of the pot. This alteration was expected to increase catch efficiency compared to that of the conical pots used by the fleet today. However, experimental fishing results showed that the modified pots had significantly lower catch efficiency than the standard conical pots, as only ~66% of the number of crabs caught by the conical pots were caught in the moon pots. The main reason for this reduced catch efficiency likely was the initial steepness of the moon pot, which may have made it difficult for crabs to reach the pot entrance. These results demonstrated that pot design can dramatically affect catch efficiency of snow crabs.

Seal exclusion devices in cod pots prevent seal bycatch and affect their catchability of cod

In order to maintain a viable inshore fishing industry, it is important to limit the incidence of seal-inflicted damage to static fishing gear and additional catch losses. Studies in the Baltic Sea have shown that this could be achieved by implementing alternative fishing methods, e.g. by using fish pots instead of lines or gillnets. However, as in net and line fisheries, a pot fishery could also result in bycatch of seals. In order to prevent these bycatches, pot entrances were equipped with seal exclusion devices (SEDs) of various sizes and shapes. A field study was conducted to investigate what effect different types of SEDs had on the bycatch of seals as well as on the pot's catchability. When pots where equipped with SEDs the bycatch of seals was reduced to zero without negatively affecting the pot's catchability. The shape and size of the entrance did have a significant effect on the pot's catchability and the size of the fish. Using symmetrical oval shaped entrances along with larger rectangular entrances divided into two smaller openings increased the pot's catchability.

The effect of two pot types on entrance rate and entrance behavior of the invasive Japanese swimming crab Charybdis japonica

The behavior of the crab Charybdis japonica towards two pot types was recorded by camera while placed in a tank. One pot was box-shaped with two slit entrances, while the other was dome-shaped with two open funnel entrances. Pots were placed in the tank and the behaviors of single crabs were video recorded for 1.5 h (30 recordings/pot). Crabs attempting to enter the box-shaped pot had trouble getting through the entrances and 37% gave up, while 100% of those reaching the dome-shaped pot's entrances easily entered. The entrance on the dome-shaped pot was 100% efficient while that on the box-shaped pot only caught 66% of the crabs that contacted it.

Measuring efficiency and predicting optimal set durations of pots for blue cod Parapercis colias

Continuous video recordings of entries and exits from blue cod Parapercis colias pots in central New Zealand showed that less than 8% of approaches to pot entrances during 30 min sets led to pot entries. Thirty-four percent of the blue cod that did enter exited before the pot was hauled. Time-courses of approaches were relatively steady throughout 30 min, though the greatest number of approaches occurred during the first 10 min of sets. A more limited dataset of 60 min sets found that the number of blue cod contained in pots changed little after 30 min, indicating that maximal catch per unit effort would be derived from sets of about 30 min duration. Local topography appeared to constrain pot entries in some situations. A spreadsheet model of catch per time as a function of number of pots, set durations, haul time and catch handling time indicated that 30–40 min sets were optimal for the range of variables we considered. It is clear that efficiency of capture might be greatly improved.

Application of the Rayleigh distribution to size selectivity of small prawn pots for the oriental river prawn, Macrobrachium nipponense

The design feature of pot entrance may affect the number and size distribution of trapped prawns. The selection process of the entrance was studied. To identify how the design feature of the funnel affects the selection process for the body length (BL), a series of tests was carried out using model pots without bait in the south-basin fishing ground of Lake Biwa, Japan. The size of trapped prawns fits a Rayleigh type distribution. The mean value of the top one third of all the individuals BL 1/3 when arranged in ascending order could be used as the principal index to evaluate the selectivity of the entrance.The results suggest that in a small pot used here, the selectivity seems likely controlled by both the pot neck height and opening diameter.

フグ篭網の漁獲機構に関する二，三の知見

The fishing mechanism of fish pots was studied through both field research and laboratory experi-ments using puffer, Fugu niphoble. Sizes of fish pots and puffer, Lagocephalus wheerleri caught were measured on board. As a first step to investigate swimming motion near the entrance to pots, the behavior of fish at the entrance to a simplified pot set in the aquarium were recorded on video tape and investigated using a motion analyzer. The following results were obtained. The turning motion of fish varied with the width of the pot's entrance “…when the ratio of the width of the pot entrance to the total length of the fish was less than 0.78.” However, no turning motion was observed when the ratio fell below 0.57. These values appear to be the upper and lower limits for the turning motion of fish in the pot's entrance. The numbers of fish caught estimated from tank experiments agree well with the field research. The fish-catching mechanism of fish pots seems to depend on the behavior of the fish inside the pot's entrance.

Catch performance of small prawn pots in terms of selectivity and escape

The size distribution of entrapped prawns in a pot may be governed by the construction of the pot entrance. The selectivity of entrances was studied. To clarify how the pot entrance design affects the size distribution of entrapped prawns in a pot, a series of tests were carried out with live prawns in a tank. In each case, the variations in the mode of sizes could be explained satisfactorily by the differences in the diameter of the opening. The escapement ratio, a could be used as the principal index to evaluate the selectivity of the entrance. The results suggest that the pots with large openings accelerate the escape of the prawns.

Relation between ingress, escape and height of pot entrance.

In this paper, attention is focused on the relation between the ingress and escape behaviour of prawns and height of pot entrance for the purpose of applying these data to investigate the catching mechanism of a small shrimp pot. The experiments were performed in a tank using six model pots without bait. The behaviour of prawns neaz the entrance funnel was recorded using video camera for a duration of 3hours per experiment. In each case, the variation in the number of individuals in a pot was well defined by the model equation. The relative catching efficiency was at maximum value near 8cm height of pot entrance.

Incidental Capture of Diamondback Terrapin by Crab Pots

Two-hundred eighty-one diamondback terrapins (195♂, 86♀) were captured in baited and unbaited crab pots during premolt crab capture studies in South Carolina. Sampling was conducted four consecutive days each week from mid April to mid November, 1979, in the Ashley River estuary, and daily from April to July, 1980 and 1981, in the Wando River estuary. Fifty-five percent of the total were captured in April, and 32% in May. Males outnumbered females 2.3∶1, but this ratio may reflect crab pot entrance bias which restricted entry of large females. Median plastron length of females was 121 mm (range 76–175) and that for males 100 mm. (range 79–128); only two male terrapins equalled or exceeded the minimum legal size limit of 127 mm. Few captured terrapins were drowned when crab pots were checked daily, and it is estimated that capture mortality amounted to 10%. Terrapin catch per baited crab pot averaged 0.16 daily in April and May. Incidental terrapin capture by 743 commercial crabbers (fiscal year 1982 numbers) in South Carolina during April and May was estimated at 2,853 daily, and mortality was estimated at 285. These estimates are exclusive of mortalities resulting from lost pots. The impact of crabbing activities is believed to pose no threat to the terrapin population at current levels of commercial terrapin harvest. Because female terrapins mature at the minimum legal size, no segment of the egg laying population is protected except seasonally. If direct harvest of terrapins ever approaches that of the early twentieth century, then it is recommended that legal size limits be changed to protect a segment of the mature female population.