A comparison of copepod egg production rates in the Gulf of Alaska Russell R. Hopcroft

What are the rates of planktonic secondary production in the Gulf of Alaska? We need to know the biomass We need to know the rates P = Biomass x Growth rate Egg production is the most commonly used method for estimating the rate of production in copepods

GoA Copepods Low diversity: ~12 species dominate Neocalanus flemingeri, N. plumchrus, N. cristatus Metridia pacifica (+M. okhotensis) Pseudocalanus mimus, P. newmani (P. minutus) Oithona similis Eucalanus bungii Acartia longiremis Calanus marshallae, C. pacificus (Centropages abdominalis)

The Problem Egg production in Pseudocalanus and Oithona species have been the focus of a number of studies Very few studies of egg production in any Metridia species Literature suggests Metridia clutch sizes and egg production rates are low, despite its frequent dominance in communities Initial “conventional” incubations (i.e. eggs screened after 24 hours) in Alaska tended to indicate low egg production, BUT incubations in small trays showed eggs laid then “disappeared” during incubations

The Question Is Metridia really any different from other copepods, i.e. does it really have low egg production?

Metridia pacifica Metridia okhotensis

Egg production towers After h, water sucked from upper tower & removed Eggs observed undisturbed as clutches where they fell 70 ml removal towers in TC plate 200 µm mesh held 5 mm above bottom 1 female per tower, setup at night

Egg viability Metridia pacifica 4-5°C8-12°C M. ohkotensis

Metridia pacifica M.okhotensis

Metridia pacificaM.okhotensis

Clutch size coupled to body size in M. pacifica, but not in M. okhotensis

Pseudocalanus mimus (& P. newmani not shown )

P. mimus 1 female per 70 ml TC flask of genera per experiment Checked daily for 3 days

P. mimus

P. newmani

Variability in Clutch size

Oithona similis

Oithona similis (egg ratio)

Summary Metridia is no production “slacker”, simply misunderstood Metridia and Pseudocalanus species have similar specific rates of egg production, Oithona similis has much lower production Egg clutch size and female body size are related, with minima in winter, increasing to peak in May, then falling slightly (tracking Primary Production cycles) Although clutch size declines after May, a greater proportion of females produce eggs each day once water has warmed; THUS secondary production in the GoA remains relatively constant from May through October