Echoes of Japanese common squid Todarodes pacificus off the Pacific coast of eastern Hokkaido

 

We measured daytime echoes of Japanese common squid Todarodes pacificus off the Pacific coast of eastern Hokkaido in 2009 and 2011. We recorded the echoes of squids with approximately 15–25 cm mantle length, occurring at a depth of 15–100 m and temperature of 3–12°C, in the water column over the continental shelf. These echoes, which ranged in height from approximately 10 m to 50 m, were distinguishable from the small echoes of the other marine organisms, which aggregated near the surface and upper layers.

Relationship between growth and biochemical constituents in the liver of laboratory-reared juvenile barfin flounder (Verasper moseri)

 

We examined a range of biochemical constituents in the liver of barfin flounder (Verasper moseri) and evaluated the growth rate using a biochemical index. The flounders were reared for 4 weeks under 4 dietary levels (0% body weight per day, 0.5% body weight per day, 1.25% body weight per day, and 2% body weight per day), and measured the total body length and body weight at weekly intervals. Further, the specific growth rate (SGR) was calculated using the equation 100 × (lnW_t-lnW₀)/d, where W₀, W_t, and d represent the initial body weight, body weight at week t (t = 1-4), and number of days, respectively. We analyzed a range of biochemical constituents, including triglyceride, glycogen, moisture, protein, RNA, DNA, and phospholipid, in the liver after feeding. From these, significant correlations between SGR and several biochemical constituents, namely, RNA/DNA ratio, phospholipid/DNA ratio, condition factor, liver index, and protein/DNA ratio, were shown. Our results indicate that the biochemical constituents represent useful biochemical indices for evaluating the growth rate of released barfin flounders in the field.

Body size of juveniles released and its effect on the return rate of chum salmon in  Shokanbetsu River, Hokkaido, Japan

 

To investigate the effect of release size of chum salmon Oncorhynchus keta juvenile on the return rate in Shokanbetsu River, northern Hokkaido, small and large body size groups of otolith-marked juveniles were released in 1996 and 1997. The mean fork length and body weight of small and large size groups at release were 49.0 mm, 0.95 g and 54.0 mm, 1.32 g in 1996, and were 46.0 mm, 0.77 g and 51.0 mm, 1.09 g in 1997, respectively. The return rates were calculated by cumulative numbers of 3-5 age fish returned to Shokanbetsu River from 1998 to 2001 divided by numbers of fish released. The return rates of small and large size groups released in 1996 were 0.105% and 0.097%, and those released in 1997 were 0.109% and 0.138%, respectively. These results suggest large body size at release is not always effective to increase the return rate of chum salmon.

Estimates of residual numbers of hatchery-reared masu salmon smolts stocked into a stream, southwestern Hokkaido

 

In western Hokkaido, recreational fishing for juvenile masu salmon is prohibited in April and May to conserve masu salmon populations. To evaluate the appropriateness of the regulation, a total of 90,062 masu salmon juveniles were stocked in Sannai River on 18 May in 2007, and the number of masu salmon residing in the river was accessed for about 1 month after stocking. The estimated number of fish (± standard error) remaining in the river was 51,516 (± 25,606) on 24 May (6 days after stocking), and 15,522 (± 6,884) on 31 May, indicating that more than 80 percent of stocked juveniles had migrated seaward by the end of May. Most of the juveniles captured on May 31 had not fully smoltified. These results suggest that the current regulation to conserve the migrating masu salmon smolts in spring is reasonable in our survey area, and most of the smolts emigrate from the river during the period when recreational fishing is closed.

Distribution and developmental process of wakasagi, Hypomesus nipponensis larvae in Lake Abashiri

 

The spatial distribution and development of wakasagi Hypomesus nipponensis larvae were examined to elucidate the early life history of this species in Lake Abashiri, Hokkaido. Wakasagi larvae were collected with a larva-net at nine stations in Lake Abashiri from May to July in 1997-1999. All fish collected were in the larval stage (Phases A-F). From May to early June, a higher density of wakasagi larvae was distributed near the waters around the river entries, which mainly consisted of newly-entered larva into the lake. These larvae soon dispersed throughout the lake because the yolk-sac larva (phase A) was found in the lower section of the lake. Horizontal distributions after their dispersion were highly aggregated, but there was no significant trend for either spatial distribution or developmental phase composition. Presumably, the spread of the larva may be a passive process with the water currents. The larval density was highest in 1997, but the developmental rate was low. Thus, larval density in Lake Abashiri affects their developmental rate.

Hatching period of sandfish (Arctoscopus japonicus) juveniles in Ishikari Bay (Short Paper)

 

Sandfish (Arctoscopus japonicus) is one of the most important commercial species occurring in the Ishikari Bay, in the Sea of Japan. We estimated the hatching period of sandfish stock by using the daily growth increments of the sagittal otolith. We showed that the hatching dates occurred from early February to early April, but the start and peak of the hatching periods differed from 2009 to 2012. These variations in the time of hatching may be caused by differences in the water temperature during the embryonic period and differences in the age composition of maternal stock.

 Fish predators of juvenile chum and pink salmon in coastal waters of Abashiri region, eastern Hokkaido (Short Paper)

Stomach contents of fish captured by gillnet, angling, and trawl in the coastal waters of Abashiri region were examined for the presence of juvenile chum salmon Oncorhynchus keta or pink salmon O. gorbuscha in May-June 2003-2005. The following 4 species were recognized as fish predators of juvenile chum and pink salmon in the coastal waters of Abashiri region; masu salmon O. masou, pointhead flounder Hippoglossoides pinetorum, kurosoi rockfish Sebastes schlegelii, and saffron cod Eleginus gracilis. In a review paper (Nagasawa, 1998), 9 species were listed as fish predators of chum salmon in Japanese coastal waters, but the latest 3 species were not included. In this paper, we newly add the 3 species, i.e. pointhead flounder, kurosoi rockfish, and saffron cod, as fish predators of juvenile chum or pink salmon.