Science Stories: Adventures in Bay-Delta Data

Feeling Crabby
  • August 29, 2022
any small crabs running around on a tray

More underappreciated data!

This is the second blog in our series on underutilized datasets from IEP.

San Francisco Bay Study’s Crab Catch dataset

Curated by Kathy Hieb and Jillian Burns

The San Francisco Bay Study has been sampling with otter trawls and midwater trawls throughout the San Francisco Bay, Suisun Bay, and Delta since 1980. Their fish data have been used in a number of scientific studies, regulatory decisions, and journal articles. However, did you know they measure and count crabs in their nets too?

Bay Study’s stations are all categorized as “Shoal” (shallow areas) or “Channel” (deeper samples). Crabs are collected by otter trawl, which is towed along the bottom of the water, scraping up whatever demersal fishes and invertebrates it comes across. Truth be told, it’s not the best way to catch crabs, because most crabs like hiding under rocks where they are out of the way of the net, but it does give us a metric of status and trends of some of the most common species of crabs, including the Pacific rock crab (Cancer productus), the graceful rock crab (Cancer gracilis, also known as the slender rock crab), the red rock crab, and everyone’s favorite, the Dungeness crab (Metacarcinus magister).

After the net has been towed on the bottom for five minutes, it’s brought on board the boat and the biologists count, measure, and sex the crabs they’ve caught (Figure 1). This can be tricky, because crabs can be FAST! Especially the smaller Dungeness crabs (Figure 2). The biologists have to be careful and pick up the crabs by their back side to avoid getting pinched by their claws, which definitely takes practice.

a large crab is held by the back of its shell and is being measured with calipers
Figure 1. Each crab is carefully measured using calipers. This is where experienced biologists have to practice holding the crabs carefully to avoid being pinched. Image credit, Lynn Takata, Delta Science Program.
tray full of several dozen small crabs
Figure 2. Lots of little crabs! Juvenile crabs can be particularly hard to catch, and particularly hard to tell apart. Image credit: Kathy Hieb, CDFW.

Once all the crabs are counted and measured, they are entered into a database that goes back to 1980. Bay's Study's Dungeness crab data have been used to help manage the commercial crab fishery because fisheries-independent data is valuable. From 1975 to 1978, an estimated 38-82% of the Dungeness crabs in the central California region rear in the San Francisco Estuary each year (Wilde and Tasto 1983). This dataset was also very helpful in tracking the introduction, expansion, and decline of the Chinese mitten crab (Eriocheir sinensis), which briefly took over the brackish regions of the estuary but declined as rapidly as it arrived (Figure 3. Rudnick et al 2003). Bay Study's crab data has also been combined with other datasets to see how the estuarine community as a whole responds to climate patterns and human impacts (Cloern et al. 2010).

line graph showing annual average catch per trawl of five species of crabs caught by Bay Study in each region of the Estuary (South Bay, Central Bay, Suisun, and the West Delta) - click to enlarge in new window
Figure 3. Annual mean catch per trawl of the most common species of crabs across each region of the estuary. Dungeness crabs are the most frequently caught, with peaks in South Bay, Central Bay, and San Pablo in 2013 and 2016. Chinese mitten crabs had a spike in abundance in Suisun and the West Delta around 2002, but are rarely caught before or after. The red rock crab, graceful rock crab, and Pacific rock crab are only caught in South Bay, San Pablo, and Central Bay, and then only in low abundances. Click image to enlarge.

However, a lot of questions remain to be asked of this dataset. Why did we see such high catch of Dungeness crabs in 2013 and 2016? What are the drivers between the lesser-studied crabs, such as the graceful rock crab? How does the salinity preference of each species of crab differ (Figure 4)? If you want to investigate these questions yourself, data are available on the CDFW file library website. But be careful, the data have a few hiccups in them, such as changes to sampling sites over time, missing samples during period of boat break-downs, and other caveats. Be sure to read the metadata and make sure you understand the data before using them.

dot plot showing the salinity at which each species of crab is caught - click to enlarge in new window
Figure 4. Dot plot showing the salinity of each trawl where each species was found from 1995-2005. The Pacific rock crab, graceful rock crab, and red rock crab mostly occur at high salinity (25-32 PSU), but the Dungeness crab is often found in brackish water (10-32 PSU), and the Chinese Mitten crab was found in fresh to brackish water and mostly absent from high salinity water (anything greater than 28 PSU). Click image to enlarge.

Further reading

Categories: BlogDataScience, Underappreciated data

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