Abundance, biomass and life cycle patterns of euphausiids (Euphausia pacifica, Thysanoessa inspinata and T. longipes) in the Oyashio region, western subarctic Pacific

Abstract

A series of oblique hauls with Bongo nets (0–1000 m) was made during the period of August 2002 through August 2004 in the Oyashio region, western subarctic Pacific, to investigate abundance, biomass and life cycle patterns of the three predominant euphausiids (Euphausia pacifica, Thysanoessa inspinata and T. longipes). While the three euphausiids occurred throughout the entire study period, E. pacifica was the most abundant (1,120 indiv.m^−2, or 832 mg C m^−2), followed by T. inspinata (163 indiv.m^−2, or 144 mg C m^−2) and T. longipes (73 indiv.m^−2, or 75 mg C m^−2). Judging from the occurrence of females with spermatophores and furcilia larvae, the spawning was considered to take place twice a year (April–May and August) for E. pacifica, year-round (peak season: March–May) for T. inspinata and in spring (March–May) for T. longipes. The population structure in terms of size (=total length) frequency distributions of the three euphausiids was characterized by the frequent co-occurrence of 2–3 cohorts in the same samples. The maximum size of males and females found were 21 mm and 24 mm, respectively, for E. pacifica, 18 mm and 23 mm, respectively, for T. inspinata, 27 mm and 31 mm, respectively, for T. longipes. Tracing the sequence of cohorts, the life spans of E. pacifica, T. inspinata and T. longipes were estimated to be 17–26 months, 17–19 months and 29–31 months, respectively. These results are compared with reports of the same species in other habitats in the light of regional variations

Immature euphausiids do not appear to be prey for humpback whales (Megaptera novaeangliae) during spring and summer in Southeast Alaska

“…In other years, whales might target immature euphausiids earlier in the year if, for example, they reach larger sizes, form denser aggregations or develop substantial lipid stores sooner. Observations of the euphausiid community during the study period, including species, spawning schedules and size structure (Szabo and Batchelder, in press), are similar to other reports for the region (Paul et al 1990) and many parts of the North Pacific (Gomez-Gutierrez et al 2005, Kim et al 2009). This suggests that in other years numerical and/or energy density may be the factors suggested to be important to foraging whales that are most likely to vary substantially.…”

“…, Kim et al . ). This suggests that in other years numerical and/or energy density may be the factors suggested to be important to foraging whales that are most likely to vary substantially.…”

Late spring and summer patterns of euphausiid reproduction in Southeast Alaska fjord waters

“…A substantial proportion (~30%) of immature T. longipes in early June had already developed to the midfurcilia stage, and by late August, furcilia were nearly absent from the study region. This suggests a 158 Species --Length (mm) -------Surveys 1 to 6 ---------- Kim et al 2009). Kim et al (2009) noted a short period of dominance of T. longipes furcilia, which is consistent with our observation that larvae become juveniles at smaller sizes in later surveys.…”

The diets of humpback whales (Megaptera novaeangliae) on the shelf and oceanic feeding grounds in the western North Pacific inferred from stable isotope analysis

“…All these species may form daytime breeding surface swarms (Smith and Adams 1988). Judging from the occurrence of females with spermatophores and furcilia larvae, the spawning was considered to take place from March to May for T. longipes (Kim et al 2009). Daytime surface swarms of T. spinifera usually occur from about April to July (Smith and Adams 1988).…”