Fisheries Techniques Week 2- Length, Weight, and Body Indices

what are the 4 most important things in a fish's life cycle?

reproduction (recruitment), body growth (somatic growth), natural mortality and anthropogenic mortality (fishing)

what 2 things add to fish population

growth and reproduction

what 2 things decrease fish population

fishing and natural mortality

why is body growth important to fish population?

bigger fish have more eggs and make more babies. they grow indefinitely but start slowing down eventually

pretend there is a bar graph that shows a downward power curve, but is mostly missing data from the year 0 and 1. The highest number of fish caught and shown on the bar graph starts at 2 and then steadily decreases. interpret why this is

- gear selectivity towards larger fish (starting at age 2). although we can't catch fish that are smaller, they must still be there because of the higher amount of fish caught at age 2.

- as age increases, fish population decreases

- conditions are so variable; hard to catch accurate numbers of fish to give a good population estimate

- fish subject to natural mortality and fishing

- year 2 is the year fish are "recruited" to the fishing gear or fishery

steps to analyze fish population

1. consider the source of data, and the biases with the data

2. consider growth and reproduction processes, then consider natural and fishing mortality

envision a bar graph of fish population, showing peaks at years 0-2, then a dip, then another peak followed by steady decrease at 12 years. interpret

- there is incomplete data

- assume standard, reliable data

- there are not enough details

- consider factors: environmental conditions super bad some years? climate change?

- stock limits? fish allowed to be caught between a certain size; above and below must be released

what is PSD (proportional stock density)?

- number of bigger fish / number of smaller fish

- says how many fish are of quality, preferred, memorable, or trophy size

- describes the relative proportion of fish of certain sizes

- used to numerically describe length-frequency data

- not a percentage, it's an absolute number

PSD equation

(number of fish above minimum quality length / number of fish above minimum stock length) x 100

what is quality length

minimum size most anglers like to catch

what is stock length

length at maturity or minimum length sampled by gear or that has recreational value

would a fisher rather fish at south iron creek (lower PSD, more fish population) or castle creek (larger PSD, less fish)

if goal is more fish go to south iron, if goal is big fish go to castle

relationship between PSD and abundance

related proportions between class sizes, not total abundance.

how are weight/length relations assessed and graphed?

- use a power curve: weight = a Length ^b

(a and b are parameters)

what are the uses of weight-length relations

1. estimate one based on the other

2. assess levels of "fatness" or fish health (condition factor)

what is the condition factor

a measure of the plumpness of fish

what is the formula for the condition factor

K = (W/L^3) * 100,000

K is fulton condition factor, W is weight, L is length (mm)

imagine a scatter plot with a line of best fit, showing a slight increase in plumpness of burbot in Arctic Rivers over the years. what does this mean and what has caused it

- climate change? increase in growing season as a result. food web changes too?

- fish are getting fat