Straight Lines In A Curved World

This weekend we had the great privilege of visiting Auntie Donnie's property mauka of the fishpond. This was an amazing cultural experience and a great opportunity to see the watershed at all its stages. The water condenses at the mountaintop and drips down into pools, where it is fresh enough to drink. The stream runs through a bamboo forest, where high flow and small rocks and sand have carved out some of the larger rocks. Once the stream gets to actual cleared forest, it empties into taro patches, where it brings nutrients to the taro and flushes the roots. The taro pull some sediment and extra nutrients out of the water, and clean water continues downstream. Historically, this tiered system of taro production was an important part of land and water use in this area, and many community and conservation groups would like to see it return to the He'eia ahupua'a. The patches provide fish as well as taro, since freshwater fish live among the stems and roots. Currently, the wetland area immediately upstream of the pond is filled with California grass (Brachiara mutica), and the water remains in pools, which flood and connect during heavy rains in the winter. Some experimental taro patches exist in this area, but removal of B. mutica and mangrove (which has extended back from the pond into the wetlands) is costly and difficult. One hope is that taro patches will be able to handle a sediment load comparable to what the mangroves are currently holding, so that mangrove removal will not overload the pond with sediment.

Now You Sea 'Em...

There is a seahorse in this picture. (Photo credit: Beth Deacy, LAIP intern)

Photo credit: Beth Deacy.

In the last few weeks, we've seen a number of seahorses in the fishpond. They're probably Hippocampus kuda (Bleeker 1852), the smooth seahorse, also sometimes called the yellow seahorse. These little guys are pretty common in Kaneohe Bay, but are usually so covered in sediment and filamentous algae that you don't notice them.

'Aumakua and "Great Change"

It turns out there are far more complex questions we can ask about the shark in the fishpond. No, I didn't see it yesterday, but we saw a good deal of thrashing near the dock and think it may have been snacking there. Let's be honest: It's cool that there is a big shark in a small area that we study. But it is a spiritual event as well as a biological one.
Auntie Donnie, professor in the UH Hawaiian Studies program and cultural teacher for the LAIP interns, gave us a little background yesterday on the shark and other spirits around the fishpond and Kaneohe Bay. When she heard there was a shark in the pond, she thought, "This is heavy." Sharks are one form of an 'aumakua, a guardian spirit. 'Aumakua have connections to certain families, so this shark is related by blood to a family here. Family members can communicate with the 'aumakua and invite them to leave or stay. Additionally, guardian spirits (kia'i) patrol the bay and make sure people are treating the land well, sharing with each other, and working hard (an approximation of the Hawai'ian word for this is pau). These spirits include Meheanu, the kia'i of the fishpond. According to legend, Meheanu brings in a lot of fish if people are pau, but can also take them away.
Auntie Donnie says the last time a shark was in the pond, a great change happened there. This could mean that the pond is already transforming, or that the kiai'i are demanding change. As researchers, we have a responsibility to honor this. So what does it mean to be pau as a scientist? We have to know our place at this site, and our responsibilities as stewards of this valuable resource. So as we look forward to great and exciting changes in the pond, we also recognize that we too have a place in making those changes.

Tiger shark in the fishpond?

Word has it, from POH, that there is a 6-foot tiger shark in the fishpond. It was last seen hanging out at one of the makahas, probably because that's the only place where it's deep enough for it to swim. According to research by Dr. Carl Meyer at French Frigate Shoals in Hawaii (Meyer 2010), tiger sharks use their individual experiences to develop habits for feeding sites, so it may either have visited the pond before, or it may come back later.

Really? How did it even get in there?
This is a good example of an organism you don't want in your pond. Ancient Hawai'ians tried to keep predators out. That said, when the pond was originally built, there probably wasn't a space large enough to let something like that in, so they didn't have to worry about the logistics of removing such a large animal.
We're going to look for it tomorrow. I wonder what it's eating...

Meyer, C., Papastamatiou, Y., & Holland, K. (2010). A multiple instrument approach to quantifying the movement patterns and habitat use of tiger (Galeocerdo cuvier) and Galapagos sharks (Carcharhinus galapagensis) at French Frigate Shoals, Hawaii Marine Biology, 157 (8), 1857-1868 DOI: 10.1007/s00227-010-1457-x

Hungry for the Fruit of the Sea: Missing Shrimp

Glass shrimp ('opae) have been present in the fishpond since I started working here. These little shrimp, which typically measure < 3 cm in length, are important nutrient shunts because they consume microalgae and detritus, grow rapidly, and are a food source for fish and invertebrates. Glass 'opae are so small that you have to dissect a lot of them in order to get enough tissue for a single stable isotope sample, and dissection requires a tiny scalpel and many hours at the microscope. In the fishpond, you can see them grazing in the shallows, and when we used dip nets or seines in the past, the nets came up covered in tiny shrimp-- so many that obtaining the 50-100 shrimp needed for a complete muscle tissue sample was not a difficult task. This past weekend, there were barely any. We used seines and dip nets at four sites across the pond, and the most shrimp we got at a single site was...twelve. This may be a result of heavy rains (salinity patterns in the water can change species distributions), or other changes in water quality. There may also be some seasonality (=change over the seasons) in shrimp populations in the pond. Seasonality in the abundance of certain species in the pond will surely affect the diets of fish and invertebrates, so it's something I'll need to investigate more in my field work.

All Eyes

P. vigil caught near the mangroves in the back of the pond.

This weekend we did a series of seines in the fishpond to catch more Thalamita crabs and glass shrimp ('opae). Usually a few seines at one site can turn up 10 crabs, but this weekend, not so. Why? We kept catching these in our nets. Meet mo'ala, or Podophthalmus vigil, the long-eyed swimming crab. The name "vigil" means "watchful," and probably refers their long eye-stalks. We found tens of these in our seines across the pond, and even a few gravid ones holding eggs. I'm not sure why they were so abundant this month. We also spotted an endemic Hawaiian species, the blood-spotted swimming crab (Portunus sanguinolentus). Surveys by Bob Hiatt in Hawaiian fishponds in the 1940's turned up these same species, which are both eaten by ulua, one of the main predators in the pond.

There is some variation in claw color in P. vigil. Notice the eyes peeking out from behind the chelipeds.

Gravid Rainbow. The panel that holds the eggs next to the body is called the apron, and is wider and rounder in females. It can hold millions of eggs at a time. This crab was caught near the seawall.

A note about Thalamita crenata or "Eek! Chemistry!"

A little background on one of my study subjects. Thalamita crenata is a swimming crab and a member of the family Portunidae. Portunid crabs are named after Portunus, the Greek god of ports and harbors, because they are often found in nearshore estuarine habitats. Their final pair of legs are modified into paddles for swimming, which makes them quick predators (and also makes them really difficult to catch). If cornered, they pack a mean pinch. Their diet is what you would expect from any detritivore: they eat whatever is available. This includes microalgae, dead fish, zooplankton...even each other.

Photo credit: inuc0r0 @blogs.yahoo.co.jp

A few weeks ago, I got back tissue stable isotope data from 30 Thalamita crabs that I caught in the fishpond. Muscle tissue from this many crabs is not enough to make any sound conclusions about their feeding habits but I noticed one interesting thing. In order to describe it, however, I need to explain some chemistry.

Nitrogen isotopes are generally a signal of trophic level, or the position of an organism in the food web. Organisms at lower trophic levels, such as algae, have lower δ15N values than organisms at higher trophic levels, like shark or ulua. This is because the heavy nitrogen isotope (15N) accumulates in animal tissues, while the lighter one is excreted. In brief: when an animal eats something, it can either absorb or excrete molecules. Chemical reactions occur more often with light isotopes than heavy isotopes. The chemical reactions that are involved in excretion (transamination and deamination) are no exception: they occur far more frequently with 14N. Therefore, when the animal produces waste, 14N is removed from the "pool" of available nitrogen in the animal. What remains, and what is absorbed, is enriched in 15N, meaning there is more heavy N isotope in the animal's tissue than there was in the thing it ate (Gannes et al. 1998). The general rule is that δ15N increases by 3‰ with every trophic level.

Why the chemical digression? My description above paints a picture of a tidy trophic ladder, with producers at the bottom and a big predator at the top. Unfortunately for ecologists, food webs are almost never this simple. Here's what I noticed in the T. crenata data from the fishpond: the crabs I sampled have a large (~3-8‰) range in δ15N values. This suggests that crabs are eating at a number of different trophic levels. Since juvenile T. crenata have the same diet as adults (Cannicci et al. 1996), these differences are not likely to be due to differences in crab maturity (young crabs aren't nibbling on algae while larger ones feast on old fish carcasses).

T. crenata is an opportunistic, detritivorous cannibal. We don't see everything these crabs eat, we can only look at the chemical clues in their tissues. As you can imagine, it is hard it is to determine what exactly a crab has been snacking on if its diet and stable isotope signatures are both so variable. Questions like this come up often in stable isotope ecology, and require us to look further, either physically by looking at the animal's stomach contents*, or mathematically by using models to predict proportions of certain foods in the animal's diet (more on this later). The variability in my crab SI values tells me already that I will need some of these extra tools in order to make sense of my data. And I thought crabs were the biggest pain about working in the fishpond...


*This is a messy but often necessary process. The polite word for it in the sciences is "gut content analysis."

Gannes LZ, Martínez del Rio C, & Koch P (1998). Natural abundance variations in stable isotopes and their potential uses in animal physiological ecology. Comparative biochemistry and physiology. Part A, Molecular & integrative physiology, 119 (3), 725-37 PMID: 9683412