Download - Preliminary demographic analysis of the shortfin mako shark, Isurus oxyrinchus

Preliminary demographic analysis of the shortfin mako shark, Isurus

oxyrinchus

Mollet H.F. and G. M. Cailliet

Contrasting VBGF and Reproductive Parameters for the Shortfin Mako

Pratt and Casey 1983 vs. Cailliet et al. 1983

Age at Maturity of Female Shortfin Mako

(when 10 and 90% are mature) 1. L (t) = 3.73 - (3.73 - 0.70) e(-0.203 t), Pratt and Casey 19832. L (t) = 3.21 - (3.21 - 0.76) e(-0.072 t), Cailliet et al. 1983

Observed and Calculated (from VBGF) Growth Rates

for the Shortfin Mako1. L (t) = 3.73 - (3.73 - 0.70) e (-0.203 t), Pratt and Casey 19832. L (t) = 3.21 - (3.21 - 0.76) e (-0.072 t), Cailliet et al. 1983

Band-Count and Capture-Date Frequency Histograms

Check on Seasonality of #Bands(based on preliminary band count data sheets)

Growth Rates of Southern California Makos from Modes of Length-Frequency

Histograms

• ~50 cm/yr at age 0.25 yr~34 cm/yr at age 1 yrFall data Cailliet et al. 1983 assuming 2 bands/year.

• ~45 cm/yr at age 0.67 yr~30 cm/yr at age 1.67 yrO'Brian and Sunada 1994, modes at AL = 42 and 53 cm (TL = 122 and 152 cm) from 2+ and 3+ year old makos, respectively.

• ~60 cm/yr at age 0.25 yr~30 cm/yr at age 1.0 yr20-25 cm/yr at age 2.0 yrHanan et al. 1993, modes at AL = 34, 44, and 54 cm (TL = 100, 130, and 150-150 cm) from 0.5, 1.5 and 2.5 year old makos, respectively.

Catch Curves Estimation of Survival from Age

Composition(Ricker 1977, Krebs 1989)

• Right limb is a survivorship curve that is both age-specific and time-specific if the following assumptions are met:

• Survival rate is uniform with age• Survival rate is uniform with time• Sample taken randomly from age

groups involved• Recruitment is constant for all age

groups

• If these 4 assumptions are violated, the right limb of the catch curve will often not be a straight line.

Catch Curve of Female Shortfin Makos

Catch Curve for Exp. Drift Longline Fishery in Southern California during 1988-1991

O'Brian and Sunada 1994

Catch Curve for Makos taken by Japanese Longliners off the eastern coast of Australia(Stevens 1992)can add n=98 females, Z = 0.61 but # to small to derive meaningful

catch curve. Could try smoothing.

Catch Curve for Shortfin Mako off East Northland and Bay of Plenty, New Zealand 1974-90 (Saul and

Holdsworth 1992) (females extracted and large females from off-shore

added)

Catch Curve for Female Shortfin Mako from western North Atlantic, n = 1113 (Casey and Kohler 1992)

Z= 0.70, too large! but why,

Catch Curve for Shortfin Mako off South-Eastern Australia 1961-90 (Pepperell 1992)

D? Mejuto for years 1983 and 1984combinend female data for the 2 years then

calculatet age_FL. Was done later!

Summary of Z's from Catch Curve for Female Shortfin Makos

Parameters for Demographic Analysis

• 1. Length at Age (VBGF).• 2. Estimate of natural mortality (M)

Hoenig 1983 (requires max. age estimate, we used 7 ln2/k = 23.9 years (k = 0.203 year-1).Pauly 1974 (requires max. size = 3.73 m TL, VBGF

k = 0.203 year-1, and water temperature T = 20 C.

• 3. Estimate of fishing mortality (F) or total mortality

(Z = M + F); we used catch-curve derived Z.• 4. Natality, to be discussed next.

Effective Natality vs. TL10, 50, 90% mature at TL = 2.60, 2.72, 2.85 m)

mx(TL) = (1/4) 0.572 TL2.66 e(-47.481+17.505 TL)/(1+e(-47.481+17.505 TL));

Effective Natality vs. Age10, 50, 90% mature at age 4.8, 5.4, 6.1yr

mx = (1/4) 0.572 TL2.66 . e(-47.481+17.505 TL)/(1+e(-47.481+17.505 TL));

with TL = 3.734-(3.734-0.702)e(-0.203 age)

Life-History Table of Isurus oxyrinchusInput: VBGF, Z = 0.30 year-1, mx = effective natality

Output: Ro = 2.0, G = 9.0 year, r = 0.076 year-1, er = 1.08

Life-History Table of Isurus oxyrinchusInput: VBGF, Z = 0.34 year-1, mx = effective natality

Output: Ro = 1.08, G = 9.72 year, r = 0.0084 year-1, er = 1.01

Life History Parameters for Isurus oxyrinchus

(F starts at age 0)


First year mortality 2x subsequent mortalities

Life History Parameters for Isurus oxyrinchususing 3-Year Reproductive Cycle

(F= 0.05 and 0.10 year-1start at age 0)


M1 = 2 x M1+j

Life History Parameters for Sharks(Adding to Cortés 1995 and 1996)

Finite Rate of Population Change (er) vs. Z(Southern Hemisphere vs. Western North Atlantic Age at

Maturity)

Finite Population Rate Decrease (er) vs. ZBase calculation with observed fecundity and southern hemisphere age at maturity