Tadashi Tokai

Selectivity and gear efficiency of trammel nets for kuruma prawn (Penaeus japonicus)

Abstract To study the effect of gear selectivity on kuruma prawn ( Penaeus japonicus ) fishing experiments using trammel nets with different mesh sizes were conducted in Ohmi Bay, Yamaguchi Prefecture, Japan. To provide comparative data the same nets were used on kuruma prawn of known size composition under controlled conditions in a water tank at the Yamaguchi Prefectural Naikai Fisheries Experimental Station. The experiments clarified the gear efficiency of trammel nets for kuruma prawn and confirmed the optimal mesh size for resource conservation. Mesh selectivity by a trammel net used for kuruma prawn shows a peaked curved, with the efficiency at the peak of mesh selectivity increasing in proportion to the mesh size. From the resulting catching efficiency data, an optimum mesh size of 42.8 mm was determined for kuruma prawn of 110–140 mm body length. This paper includes a method for estimating the efficiency of gillnets, which is an extension of Kitaharas method.

East Asian seas: A hot spot of pelagic microplastics.

To investigate concentrations of pelagic micro- (<5mm in size) and mesoplastics (>5mm) in the East Asian seas around Japan, field surveys using two vessels were conducted concurrently in summer 2014. The total particle count (pieces km(-2)) was computed based on observed concentrations (pieces m(-3)) of small plastic fragments (both micro- and mesoplastics) collected using neuston nets. The total particle count of microplastics within the study area was 1,720,000 pieces km(-2), 16 times greater than in the North Pacific and 27 times greater than in the world oceans. The proportion of mesoplastics increased upstream of the northeastward ocean currents, such that the small plastic fragments collected in the present surveys were considered to have originated in the Yellow Sea and East China Sea southwest of the study area.

A method of determining selectivity curve of separator grid

This paper describes a theory and method for determining a grid selectivity curve from the data of a fishing experiment using a grid separator with several bar spacings. Grid selectivity is defined as the probability that a fish does not pass through a grid given that it has encountered the grid. Assuming that grid selectivity is regarded as a sieve process, grid selectivity, sg, can be expressed as a function of the ratio of cross-sectional diameter to bar spacing as follows: sg(d,L) = Sg(R), where L and d are the cross-sectional diameter of the fish body and bar spacing of the grid, respectively, and R = Ld. Application of the model is demonstrated by a fishing experiment using three grids with bar spacings of 8, 10 and 15 mm, carried out in a shrimp beam trawl in the Inland Sea of Japan. Grid selectivity was plotted against length by bar spacing, of two shrimp species (southern rough shrimp Trachypenaeus curvirostris and mantis shrimp Oratosquilla oratoria), two crab species (charybdid crab Charybdis bimaculata and portunid crab Portunus hastatoides), and frog flounder Pleuronichthys cornutus. The selectivity analysis in terms of the non-dimensional parameter R revealed that, for each species, a single selectivity curve describes accurately data of each of the grids as a master curve of grid selectivity. The master curve of grid selectivity allows estimation of the selectivity curve of grids other than those tested in this experiment.

Microplastics in the Southern Ocean

A field survey to collect microplastics with sizes <5mm was conducted in the Southern Ocean in 2016. We performed five net-tows and collected 44 pieces of plastic. Total particle counts of the entire water column, which is free of vertical mixing, were computed using the surface concentration (particle count per unit seawater volume) of microplastics, wind speed, and significant wave height during the observation period. Total particle counts at two stations near Antarctica were estimated to be in the order of 100,000pieceskm-2.

Size selectivity of escape holes in conger tube traps for inshore hagfish Eptatretus burgeri and white‐spotted conger Conger myriaster in Tokyo Bay

In Tokyo Bay, conger tube-trap fishers voluntarily deployed escape holes of at least 13 mm diameter to reduce the by-catch of young conger smaller than the marketable size of 35 cm total length. Comparative fishing experiments in Tokyo Bay were carried out using tube traps with seven hole diameters (3, 9, 13.5, 14, 15, 17, and 19 mm), to obtain the size-selectivity of the escape-hole size for inshore hagfish Eptatretus burgeri and white-spotted conger Conger myriaster. On the assumption of the geometrical similarity in the combination of body g and hole perimeter m at a given retention probability, a single master curve s(R)=exp(−13.52+11.31R)/(1+exp[−13.52+11.31R]) was estimated for the two species in terms of relative R=g/m using the Share Each LEngth’s Catch Total (SELECT) process based on a multinomial distribution. Almost all conger and hagfish with a girth equivalent to the hole perimeter were able to pass through the hole. Thus, the two species have a high ability to escape through a narrow space by squeezing the body through the hole in the rigid PVC tube. Optimum escape-hole size is discussed from several objectives for conger tube-trap fishery management, e.g. reduction of hagfish by-catch. This study also specifies the conditions for pooling data when fishing experiments using more than two fishing gears are replicated.

Effect of tooth spacing on the contact selection and available selection of a dredge for the equilateral Venus clam Gomphina melanaegis

The effect of various tooth spacing on the contact selection and the net-mouth available selection of a dredge for the equilateral Venus clam Gomphina melanaegis is evaluated by applying a contact-probability model based on the SELECT method to the data of paired-gear tests with a control dredge of 12-mm tooth spacing and four test dredges of tooth spacing 16, 20, 24 and 35 mm. A master selection curve was estimated to be s(R)=exp(−11.23R+4.799)/(1+exp[−11.23R+4.799]) where, R is l/d, the ratio of the shell length, l, to the tooth spacing, d. The shell width of 50% retention was slightly smaller than the tooth spacing, which means that size selectivity of teeth was a type of contact selection which was dependent on the tooth spacing. Contact probability, δ, of the clams coming into contact with the teeth were 0.625, 0.467, 0.563, 0.670 and 0.976 for tooth spacing 12, 16, 20, 24 and 35 mm, respectively. This indicates that some clams entered the dredge bagnet without contacting the teeth as the sand clogging the tooth spacing carried them into the bagnet. Available selection curve of the dredge net-mouth with teeth was obtained as δ s(R)+1−δ, and examined in terms of the legal landing size of the Venus clam.

A preliminary study on quantitative relations among growth, reproduction and mortality in fishes

As a quantitative approach to the life histories of fishes, the present paper attempted to predict a relation among reproduction, growth and mortality numerically with a technique of control theory, the discrete maximum principle. A method for predicting the relation was derived on the postulate that natural selection maximized the net reproductive rate subject to a few constraints. The derived method was applied to Atlantic cod and Atlantic herring populations in the Southern Gulf of St. Lawrence as numerical examples. The examples demonstrated that the theoretical reproductive effort and body weight were well consistent with the observed ones every age but the theoretical survival rates were slightly different from the observed ones. For the reasons mentioned below, however, it should be interpreted that the examples rather support the adopted postulate to a certain degree. First, in general, it is very difficult to obtain good estimates of the rates with traditional methods. Second, intense fishing pressure possibly changes the life history parameters to some extent in fish populations. Moreover, the examples also suggested that, to examine the postulate in further detail, similar analyses had to be made with the data of many fish populations on which intense fishing pressure had not been exerted.

Modelling the contact probability and size-selectivity of toothed dredges

To estimate fishing gear selectivity of clam dredges from data of paired-gear tests in the SELECT analysis process, this paper presents a statistical model of the probability of a clam coming into contact with the dredge teeth and size-selectivity of the control gear. The net-mouth available selectivity is defined as the product of the probability of a clam contacting the dredge teeth and size-selectivity induced by tooth spacing. The model based on the SELECT analysis was tested by using data generated by virtual paired-gear test in which the control dredge of the smallest tooth spacing is assumed to be size-selective. No clear difference in shell length distribution between dredges of different tooth spacings was found in the simulation when the contact probability was small. The plots of proportion of clams caught in the test dredge to the total catch number of clams were U-shaped, and the model fitted the data well. For each simulation consisting of 500 replications, the sample mean and mean square error of each parameter were obtained to evaluate the performance of estimation by comparing with the true value. Parameters were properly estimated with the model.

Method for estimating buoyancy of midwater float required to standardize hook depth in pelagic longline

A midwater float is a small float attached to the mainline of pelagic longlines to standardize the hook depth. In this study theoretical equations are presented for estimating the buoyancy of the midwater float required to lift the joints of the midwater float line on the mainline to the target depth. Sea trials using full scale tuna longline gear with midwater floats were carried out in the Indian Ocean in December 2004 and 2005, in order to examine the validity of the theoretical equations. In the sea trials, two types of midwater float settings, single midwater float setting and double midwater float setting, were tested and compared with the conventional setting. As a result, the joints of the midwater float line on the mainline were successfully lifted to the target depth as expected, demonstrating the validity of the theoretical equation. The range of hock depths in the midwater float setting was less spread over depths than in the conventional setting, and therefore, use of long float lines (100 m) with the midwater float setting allows all hooks to avoid entering the sea turtle habitat of shallower than 100 m depth. Factors affecting shoaling of the longline with the midwater float are also discussed.

Development of automatic system for monitoring fishing effort in conger-eel tube fishery using radio frequency identification and global positioning system

In this study, we developed a new automatic system, comprised of a radio frequency identification (RFID) system and global positioning system (GPS), for monitoring fishing effort, and effectiveness of effort, in fisheries using many pieces of fishing gear, such as hooks of longlines. The outline of this system is as follows. A single RFID tag with an identification (ID) number is attached to each piece of fishing gear. The RFID tag on the fishing gear passes the antenna of the RFID reader before being shot into the sea and after being hauled up from the sea. Data on the time and geographic location are measured by the GPS and recorded along with the ID number in a personal computer (PC). When a fish is caught, it is brought close to the fish-sorting table. Then, the second antenna of the RFID reader set at the side checks for a catch in the gear. The advantage of this system is that the fishing operation data can be collected without interfering with the fishermen’s usual work. The prototype of the system was tested at a conger-eel tube fishery in Tokyo Bay and was able to record data on the time, the location and the catch of individual conger-eel tubes successfully.