Hawaiian shrimp and prawn biology, and their role in vertical carbon flux Yasha Podeswa

What are shrimp and prawns? Both crustaceans from the Order Decapoda Shrimp: – Suborder Pleocyemata, Infraorder Caridea Prawns: – Suborder Dendrobranchiata

Why study pelagic Hawaiian shrimp and prawns? Biology of most local shrimp and prawn species undescribed – Morphometrics (size/shape), sex ratios, fecundity (reproductive rate of an organism/population), diet/position in food web Little commercial harvest, but may be harvested in the future – Pre-harvest data important but often unavailable – Understand the “baseline” Locally very abundant, could play a large role in vertical carbon flux through active vertical migrations

Goals of my study Describe the biology of all pelagic shrimp and prawns in the open ocean near Hawaii, including surface, mesopelagic, and migratory populations Estimate the contribution of migratory shrimp and prawns to vertical carbon flux

Why has nobody studied this before? Fish generally well studied Zooplankton generally well studied Micronekton generally not well studied – Small but bigger than plankton, free swimmers – Avoid plankton nets, too small to be caught by most fishing nets

Methods Used 3 different sampling gears with different mesh sizes – Cobb trawl (10 mm) – Isaacs-Kidd Midwater Trawl (5 mm) – Hokkaido University Frame Trawl (3 mm) 58 casts, sampled at day and night, sampling both surface scattering layer (roughly top 100 m) and deep scattering layer (roughly top 500 m)

Methods Classify all shrimp and prawns in the samples – Many samples over 50% shrimp and prawns by volume – 32 species identified from 8 families Measure carapace length, total length, wet weight and dry weight Sex individuals based on appendix masculina (for shrimp) and petasma (for prawns) Determine fecundity through counting eggs and oocites at various stages of development Determine diet/food web location through gut content analysis and stable isotope analysis

Stable isotope analysis Gut content analysis – Underestimates easily digested prey – Often very difficult to identify prey to species level – Only contains very recent meals – Hard to get quantitative data Stable isotope analysis – Quantitative data – No bias towards hard to digest prey – Integrates diet over a longer timescale

Stable isotope analysis “You are what you eat” – Predator should have similar isotopic makeup of prey Measure the amount of heavy isotopes of carbon ( 13 C) and nitrogen ( 15 N) in muscle tissue, calculate the ratio of the heavy isotope to the lighter, more common isotope   C = () 13 C 12 C sample C 12 C standard

Stable isotope analysis  13 C and  15 N measured through mass spectrometry The predator’s  15 N signature will generally be about 3.2 ‰ greater than its prey’s The predator’s  13 C signature will be very similar to the prey’s, only about 1 ‰ greater

Stable isotope analysis Why the enrichment? – Lighter isotopes ( 12 C and 14 N) are preferentially metabolised – Heavier isotopes ( 13 C and 15 N) are retained, and are thus enriched in the predator’s tissues – Small enrichments from prey to predator in  13 C allow the source of primary production to be easily identified – Larger enrichments from prey to predator in  15 N allow trophic levels to be distinguished more accurately

Stable isotope analysis Simple food chains can be identified visually Complex food webs can be estimated using mixing models

Vertical carbon flux The oceans have absorbed about 48% of total fossil fuel and cement manufacturing CO 2 emissions since the beginning of the industrial revolution The ocean’s carbon reservoir dwarf’s the carbon reservoir in the atmosphere – Over 1,000 Gt C in surface waters – Roughly 38,000 Gt C in mid and deep waters – Roughly 78,000,000 Gt C in ocean sediments – Roughly 600 Gt C in the pre-industrail atmosphere

Vertical carbon flux The ocean is density stratified – Not well mixed outside of the surface layer (upper few meters to upper few hundred meters) Mixed layer shallow in the tropics, simple turbulence not responsible for much vertical carbon flux Carbon is sequestered in the ocean primarily through the “solubility pump” and the “biological pump”

Vertical carbon flux Solubility pump – CO 2 more soluble in cold water than warm water – More dissolved CO 2 in cold, high latitude waters than warm, low latitude waters – Deep water forms at high latitudes, thus pumping high CO 2 water from the surface to the deep ocean Not very significant in the tropics, not a down welling region

Vertical carbon flux Biological pump – CO 2 fixed into organic C through photosynthesis – Sinks as “marine snow,” aggregates of dead or dying phytoplankton, dead or dying zooplankton, faeces and mucus – The carcases of larger animals (such as whale carcasses) can also contribute to vertical carbon flux – Active vertical migration can also be important

Vertical migration Virtually all zooplankton and micronekton in the oceans perform diel vertical migrations – Spend the night at the surface, and the day at depth – Some also perform seasonal vertical migrations

Vertical migration Driving force behind vertical migrations still somewhat debated, but generally seen to be based on the following factors: – Surface food quality and quantity – Visual predator avoidance – Metabolic gains – Avoidance of UV damage

Active carbon transport Some carbon is transported downwards through formation of faeces at depth – Short gut clearance times for many zooplankton make this of minimal significance, but for larger shrimp and prawns it could be more significant Much carbon is also transported through respiration of CO 2 and excretion of DOC

Active carbon transport Previous studies tend to focus on smaller migrators, especially krill – Generally make up the bulk of migratory biomass, but pelagic shrimp and prawns much more abundant in this ecosystem – Krill are generally shallow migrators (top 100 m), shrimp and prawns migrate much deeper, could contribute much more to vertical carbon flux if present in similar biomass

Active carbon transport Previous studies focusing on krill and other zooplankton migrators in the upper 100 m have shown them to contribute significantly to vertical carbon flux, ranging from 6 – 34% of POC flux Fluxes due to the less abundant micronekton have generally been ignored, but may be very locally significant in this ecosystem due to high relative biomass, deeper migrations, and longer gut clearance times

Active carbon transport My preliminary findings show that many Hawaiian shrimp and prawns migrate to deep depths (to around 500 m), and only a fraction of the population migrates to the surface each night – Thus spend numerous days/hours at depth for every day/hour at the surface, facilitating carbon transport

Wrapping up... Understanding the role of pelagic shrimp and prawns in the ecosystem through studying their biology, diet, and their contribution to vertical carbon flux should aid in any management decisions in the future, especially should this become a commercial fishery